i'm 19, but when i was 18 i date a guy who was 5 years older than me, i felt the same way you do.
who cares.
you're 18, it's a great age. apparantly he likes u for who u are anyways if he's with you, hm? 18 and 26 isn't too too bad. 14 and 20, now that's a bit different. you said yourself you're mature for your age. don't worry bout it

have fun!
~Sarah~