My theory about looking younger relates to autistic problems processing non-verbal communication, such as body language and facial expressions. To my mind, it stands to reason that a person who has problems processing such information, for example difficulties reading other people's facial expressions, then they likely also have problems communicating in the same manner, i.e. communicating non-verbally, expressing their emotions and feelings and conveying messages through facial expressions.
I think Aspies/people on the spectrum looking younger than their chronological age, especially as they get older, is because they don't acquire so many of the wrinkles and age markers that NTs do. A lot of wrinkles are associated with facial expressions, crows feet relate to laughing and smiling, frown lines are self explanatory.
So I think the difficulties relating to facial expressions is a two-way thing, and the inability or developmental delay to learn or actively acquire those skills means that Aspies don't make the facial expressions as often or to the extent NTs do, which means they don't look as wrinkled, and therefore look younger.
Of course, there might (also) be some biological/physiological reason relating to skin elasticity or something dermatological, but my theory is that it's a physical manifestation/side effect of those underlying communication problems.