(2-part post):
There are lots of reasons why you could appear older than you are. Firstly, of course, it could be purely physical. That's not necessarily a bad thing. When I was at college, a fellow student was (because of an hereditary condition) 'going grey' at 18 years old. By the age of 20 his hair was totally silver. All of the women were falling at his feet because he looked so 'distinguished'. Looking older is definitely an advantage at times!
Equally, it could be your attitudes. I used to work with a guy who was constantly going on about how young people no longer understood what good music sounded like, and saying things like "it was never like that in my day". How old was he? 60? 50?, No, he was 19!!! (So, perhaps your attitudes are tending towards the stereotypical 'grumpy old man'?).