Age appropriate is one of those phrases I've got a complicated relationship with.
On the one hand, I'm an educator (I teach math both online and offline, working with students from about 4th grade through college. I've had 8-9 year old students and I've had students older than me. Plenty of expectations I have for my college students are not age appropriate for my 8-9 year old students. I'm not going to expect 8 and 9 (or really 10 or 11) year old students to sit in one place and focus for 90 minutes straight with little to no humor. (I don't think it's a great idea with the college students either, but I think they are more likely to be capable of it.)
Because of that, I think the concept of age appropriateness can be useful for defending children from unreasonable expectations.
On another hand, I'm Autistic, and I interact with other Autistic people. I know how "age appropriate" can be used as a weapon against us, and there are a couple ways this happens.
Way the first: The concept of age appropriate is used to control what we are "allowed" to show interest in. That is, when an autistic teen or adult shows interest in something aimed at younger children, we might get "redirected" to a more "age appropriate" interest, which is one more piece of the pattern where we're not really allowed to like things, at least not safely. (Other pieces of that pattern are having everything we admit to liking used as a reward for acting more neurotypical or taken away when we act autistic/do something the staff doesn't like.)
This can also go in the other direction, where a person is told they aren't old enough for whatever they're interested in. (I had a teacher who was very concerned that I had the math interest and ability to be using exponents and roots in first grade, and there were attempts by the school to get me to stop doing math that was too advanced to be "age appropriate." I don't think this direction (alone, at least) is as common as the other, but it exists.
Way the second: The concept of age appropriate is used to "show" that we have a "mental age" corresponding to whatever interest of ours has the youngest target audience. Here, the interest of the teen or adult in the (usually) "younger" topic is used as evidence against their competence. Rather than being a teen or adult with an interest (like how the graduate supervisor at the technology help desk really liked My Little Pony and was also known to be a graduate student who knew how to solve computer and internet problems), they are treated as children in a teen or adults body. It's a pretty gross concept. (Even if you're working with the tools typical of a younger person, you've got more experience working with them -- see this crayon art as an example!)
These two problems get combined as well. There are two steps here. First, an interest that's more common among younger people is used as evidence of "mental age." Then, this arbitrary mental age/developmental level is used to restrict what else the person is allowed to show interest in, or what else they are exposed to. An example of this would be a student who enjoys Blues Clues not getting access to the general curriculum for their age, because someone who likes Blues Clues must actually be on level with a pre-schoooler. I use Blues Clues as my example because I liked the show well into middle school, and no that did not make me secretly a five year old in a middle schooler's body. This also gets used as justification for not providing education on sexual or reproductive topics, since an actual 3-5 year old shouldn't need to know that stuff yet.
The "ages" chosen for this purpose are arbitrary. Remember that interests have ranges of ages where they are more common, and remember that a person can have multiple interests, with different intended audiences between them -- whichever age is most convenient to use can probably be "justified" in this manner. Besides, "uneven" development, in comparison to the order skills and interests usually develop for neurotypical people, is pretty much a hallmark of autism. I haven't had a single coherent (neuronormative!) developmental level since I was about 6 months old, and I don't think that's unusual! That means we're essentially using a trait of autistic development, that we don't follow the same paths or patterns neurotypical people do, in order to show that we're actually small children. (Ever notice that they don't do this with the (neuronormatively) most advanced of our skills? Anyone argue I was really older because I could do more advanced mathematics? Nope!)
But back to something like the first hand, the idea of things being age appropriate or not can be used as a defense against unreasonable expectations for disabled kids, and as a defense against inappropriate therapies. For example, intensive behavioral intervention (IBI) and things based on applied behavioral analysis (ABA) often expect 40 hours a week of just the one therapy out of kids who are 2-5 years old. That's not age appropriate. Sometimes, pointing out that it's not age appropriate to expect any kid that young to manage such a schedule gets people to think about their expectations.
When I hear age appropriate in the context of autism or general disability, I don't expect that it's going to be used as a defense against ableism. Usually it's going to be the ableism (control of interests, evidence of mental age, and combinations of the two.) But I think it's important to remember that (and how) we can turn the concept around to defend ourselves from the ableist nonsense it usually justifies.
On the one hand, I'm an educator (I teach math both online and offline, working with students from about 4th grade through college. I've had 8-9 year old students and I've had students older than me. Plenty of expectations I have for my college students are not age appropriate for my 8-9 year old students. I'm not going to expect 8 and 9 (or really 10 or 11) year old students to sit in one place and focus for 90 minutes straight with little to no humor. (I don't think it's a great idea with the college students either, but I think they are more likely to be capable of it.)
Because of that, I think the concept of age appropriateness can be useful for defending children from unreasonable expectations.
On another hand, I'm Autistic, and I interact with other Autistic people. I know how "age appropriate" can be used as a weapon against us, and there are a couple ways this happens.
Way the first: The concept of age appropriate is used to control what we are "allowed" to show interest in. That is, when an autistic teen or adult shows interest in something aimed at younger children, we might get "redirected" to a more "age appropriate" interest, which is one more piece of the pattern where we're not really allowed to like things, at least not safely. (Other pieces of that pattern are having everything we admit to liking used as a reward for acting more neurotypical or taken away when we act autistic/do something the staff doesn't like.)
This can also go in the other direction, where a person is told they aren't old enough for whatever they're interested in. (I had a teacher who was very concerned that I had the math interest and ability to be using exponents and roots in first grade, and there were attempts by the school to get me to stop doing math that was too advanced to be "age appropriate." I don't think this direction (alone, at least) is as common as the other, but it exists.
Way the second: The concept of age appropriate is used to "show" that we have a "mental age" corresponding to whatever interest of ours has the youngest target audience. Here, the interest of the teen or adult in the (usually) "younger" topic is used as evidence against their competence. Rather than being a teen or adult with an interest (like how the graduate supervisor at the technology help desk really liked My Little Pony and was also known to be a graduate student who knew how to solve computer and internet problems), they are treated as children in a teen or adults body. It's a pretty gross concept. (Even if you're working with the tools typical of a younger person, you've got more experience working with them -- see this crayon art as an example!)
These two problems get combined as well. There are two steps here. First, an interest that's more common among younger people is used as evidence of "mental age." Then, this arbitrary mental age/developmental level is used to restrict what else the person is allowed to show interest in, or what else they are exposed to. An example of this would be a student who enjoys Blues Clues not getting access to the general curriculum for their age, because someone who likes Blues Clues must actually be on level with a pre-schoooler. I use Blues Clues as my example because I liked the show well into middle school, and no that did not make me secretly a five year old in a middle schooler's body. This also gets used as justification for not providing education on sexual or reproductive topics, since an actual 3-5 year old shouldn't need to know that stuff yet.
The "ages" chosen for this purpose are arbitrary. Remember that interests have ranges of ages where they are more common, and remember that a person can have multiple interests, with different intended audiences between them -- whichever age is most convenient to use can probably be "justified" in this manner. Besides, "uneven" development, in comparison to the order skills and interests usually develop for neurotypical people, is pretty much a hallmark of autism. I haven't had a single coherent (neuronormative!) developmental level since I was about 6 months old, and I don't think that's unusual! That means we're essentially using a trait of autistic development, that we don't follow the same paths or patterns neurotypical people do, in order to show that we're actually small children. (Ever notice that they don't do this with the (neuronormatively) most advanced of our skills? Anyone argue I was really older because I could do more advanced mathematics? Nope!)
But back to something like the first hand, the idea of things being age appropriate or not can be used as a defense against unreasonable expectations for disabled kids, and as a defense against inappropriate therapies. For example, intensive behavioral intervention (IBI) and things based on applied behavioral analysis (ABA) often expect 40 hours a week of just the one therapy out of kids who are 2-5 years old. That's not age appropriate. Sometimes, pointing out that it's not age appropriate to expect any kid that young to manage such a schedule gets people to think about their expectations.
When I hear age appropriate in the context of autism or general disability, I don't expect that it's going to be used as a defense against ableism. Usually it's going to be the ableism (control of interests, evidence of mental age, and combinations of the two.) But I think it's important to remember that (and how) we can turn the concept around to defend ourselves from the ableist nonsense it usually justifies.