When I read that you asked about what makes someone look young, it sparked my philosophical side lol
I wanted to know, too, so I did some deep thinking...
I thought about babies vs seniors, because that's the ultimate difference in youth. The most obvious, to me, is their skin. You attribute soft, smooth skin with babies, and wrinkled skin with old people. Also, children have "baby fat" that gives them an overall softer appearance. There's no harsh lines on a healthy kid. Especially in facial features, kids have rounder faces than adults. My sister is probably a little over 5 ft tall right now and guess how old she is? 11. If you saw her from the back, you'd probably think she was 15 or something. But when you see her face, it's really round. She has really soft lines all over, but she's not fat, she's just a little kid.
As you get older, your baby fat goes away and your muscle tone comes out more. Your skin gets tighter as you fill in with bone mass and muscle, and you start getting sharper features. There's lots of short adults out there, and when you look at them, usually you can guess their age and be in the right ballpark. My mom is actually pretty short (compared to her daughters and husband
), she's like 5'6", but nobody would ever think that she's 15 years old. She's in her early 40s, and she looks her age or a few years younger.
Anyway, you probably just have really soft features for your age and that's why people think you're so young. I have really fat cheeks and I'm not toned at all...but my height and body type gives me away