"But you look so young!"

   I'm turning 20 in a week from tomorrow. Yay me, right? Yet another milestone is behind me and I now have the privelege to buy liquor, how fantastic (I'm joking...). But something keeps bothering me, quite a lot.
   After I turned 18, whenever I got asked the question "how old are you?" and I'd answer truthfully, people would always be surprised. "But you look so young! Like 15, 16 at most." And originally, it wasn't a problem, since I would just jokingly tell them that it's great, since when I'm having my midlife crisis at 40, I could calm myself by knowing I still looked like 36. However, now it's getting a little bit annoying, because I've been hearing it for almost two years... I know I look tiny, okay?! 
   And I'm not even sure why people keep telling me this. What makes me come off like a teenager still? Is it the hair, is it my behavior, is it because I'm jittery and energetic? Maybe it's because I'm so short? (Which people also keep pointing out for some reason, despite me being only a few centimetres below average. I'm not that short! Please stop it.)
   I'm not saying that if you point out that people are short, tall or just look younger/older than they are, you are an asshole. It depends a lot on the way you go about talking about it, I for example don't appreciate being called "tiny" by people who are taller than me, or older, or when people bend down to talk to me.
   And besides, if someone really does look young for their age, or is taller/shorter than you're used to, they probably get to hear it a lot. So maybe don't point it out every time you see them, because it does get annoying after a few times.
This has been a PSA. Thank you.