I am 40 but I look like I'm in my late teens. I've heard slow visual aging is common for aspies.
My theory - based on at least 3 minutes of idle thought- is that a lot of our judgement of age is based on wrinkles on people’s faces. But you get those wrinkles by smiling, frowning, and making other expressions.
And (I assume that) autistic people make those expressions less than other people. That would make them look younger than they actually are.