Perhaps it’s more to do with being slightly “immature” in hard to identify ways.

What I mean is that people on the spectrum tend to… not recognize or not care that their behaviors aren’t “mature adult” stuff. It’s all just window dressing anyway, and if it doesn’t really matter, why put in the effort to conform? It’s just another mask.

So you’ll see a lot more visible emotion, not the strict regulation expected of adults. You’ll see a lot more action out of the body than you would with a stoic neurotypical adult. You’ll see a more rigid dress aesthetic that’s probably at least somewhat frozen in whatever time period the person figured themselves out, and thus isn’t the aesthetic of their temporal peers.

Stuff like that makes a person seem younger without necessitating looking any different, tho coupled with the underuse of expression making the face also look younger, it would certainly be amplified.