Trigger Warning: Infantilization of people with disabilities
If we recognized that children are people, that children have thoughts and feelings and needs, that in any conversation about what should be done about a child (a sign that we're already looking at this wrong when we're doing something about a person...) the goals the child has actually matter, that there is such a thing as child abuse, that there are limits on what is and is not acceptable to do to a child all need to be thought about before the convenience of the adults, the whole mental age thing wouldn't get one whit more accurate. It would probably get less creepy, since being seen like a child is seen wouldn't imply the same loss of control over one's own life that it does now, but it would be just as inaccurate as it ever was.
A developmentally disabled twenty-year old might not be able to communicate her emotions in speech any better than the kindergardener next door, but that doesn't make her somehow equivalent to a kindergardener. There's still fourteen or fifteen years of life experience, fourteen or fifteen years of maturity, between the two. Even if there are problems communicating these differences, they are still there. We didn't somehow stop developing emotionally when we were five or six and just get stuck there.
We don't somehow have only the intelligence of a toddler, and a toddler is not somehow unable to comprehend what happens around them either!
We're not somehow children trapped in adult bodies, not any more than you are. (Yeah, I know- sometimes you might feel like you didn't really mature and that what on earth are you doing in the adult world you're not ready! We might have that experience too, but it's not evidence that we actually are still kids any more than it is evidence that you really are still a kid. Be consistent, here.)
When I can't speak, I don't suddenly become five months old again. (Yes, I was talking at six months. It happens.) When you get laryngitis and can't talk for a few days, even, no one thinks that you are whatever age you learned to talk again inside. No one considers you an infant for it. When I lose speech, it's not based on a sore throat, but the effect on parts of my thinking other than "make the words I am thinking actually come out using my vocal cords and mouth" are unaffected. (They might have been affected by whatever caused me to lose speech... but generally, once my brain gives up on that I've got enough energy to keep everything else running normally. As far as my other mental processes go, I'm probably better off losing speech than not, since the energy has to come from somewhere and if I'm at risk of losing speech, I'm already pretty overloaded and something has got to go.) The point is, the rest of my mind is working fine. The reasons may be very different, but the effects aren't as different from laryngitis as you might expect. Consistency says that the reactions to each shouldn't be so different, then. Niether somehow reflects on the "mental age" of the person it happens to. It doesn't work that way when the reason for the inability to speak is a motor control issue, either. It's still not a reflection of mental age. It never was, and it never will be. People just sometimes act like it is, and that's wrong, both factually and morally.
If we treated children like autonomous beings who just needed a little more help, it might not be as creepy to make this sort of argument, and there might not be as much of a moral issue with the whole concept, but it would still be factually inaccurate, and I'm pretty sure it would still need to go. Those added factors just make it worse.