yea dude ive got the same "problem", its not much to overcome, the way i get over it is dress older, when u go out, wear long sleeve shirts and role the sleeves up, this will make ur shoulders look bigger and also makes u look older(its worked for me), plus dont cut ur hair short, thats 1 of my problems, if its 2 short i look HEAPS younger (it shows ur face alot more, so theyll notice how "young" and "imature" it really is), but once i grow it out a bit, and style it a certain way i actually pass for my real age, its how u present urself, theres alot of things u can do with your hair and clothes to make yourself look older
baggy clothes a NO NO!, that would kill everything, a nice pair of jeans, long sleeve shirt rolled up sleeves(make sure its a tight fit), cool hairstyle and good body language goes a LOONGG way!
plus if a girl says anything 2 u, just look at her and go "....ahh yea? it hasnt stopped me from anything b4" or just go "well when im 25 ill look blah blah blah" whateva age and say back "your (her age), and u already look it, so id give u a few years b4 ull start wishin u had i have

" ive said it and its worked heaps!