He tells me I look older for my age.

I would take it as a compliment, at our age (im 16 3/4) when soemone says we look older, it usually means that we look mature and more set in life meaning were out of high school and like taking care of ourselves. If its someone that knows your age, its definitely a compliment then. He probably means that you take good care of yourself and you give yourself a good image.

I'm sure it wasn't meant as anything negative but it also wasn't a good choice of words in giving you a complement. He could have just said that. like if you're 16, "you look and act so mature, like a senior. or like you're 18". That'd made you feel smarter and given you a big confidence boost. Also, maybe he said that because he kinda likes you and was hinting that you're not too young for him.

you should ask him right after, is that a good thing or is that what you like and some people do look older than their age. he might mean you look stressed or maybe he just means you seem more mature and he didn't know how to phrase it more positively