I can sympathise somewhat,
upto about age 14 I actually looked older for my age,
...but from about 15 things changed and i started to look young, ..
i'm 23 now, ...and most people on appearence just automatically assume i'm a teenager around 17 ish.
gets annoying when you go for a job and people keep asking "is this your first job?" ..
people are usually surprised when they learn my true age or think i'm BS-ing them.
but the thing that annoys me most ..is how this effects girls.
in terms of looks ..without being arrogent ..i am on the goodlooking side ....and sometimes (it dosen't happen all that often ..) i'll be walking down the street ..and i'll overhear a comment, see a girls body language suddenly change or see a girl nudge her mates, ...then they all turn and look at you and it's clear it's because they find you attractive, ....but whenever this happens they always seem to look too young like they're 14 or something..
girls who look my age (23), ..i don't even think i'm on thier radar to be honest.
i think girls are most attracted to guys a little bit older than themselves so that probably figures..
dunno what to suggest for you OP other than if you can grow some facial hair try and keep it,
also try to act more mature than your average 18 year old. you are 18 right?
i notice when i act more mature ..and people then find out my age ..they are still generally surprised but they can actually see it because i'm acting more mature