Haha there's no way you look as young as me. I graduated from college a year ago, and I'm working full-time, but I still get confused for a high-schooler or freshmen. I'm Asian, 5'5, and 22, and I can't grow much facial hair either.
http://i91.photobucket.com/albums/k303/FiveStarAzn510/IMG_0091-1-1.jpg?t=1251599205
http://i91.photobucket.com/albums/k303/FiveStarAzn510/IMG_0652.jpg
Some tips I have for you, as I was in a similar position a couple of years ago.
1. Hit the Gym! I work out 3 times a week, it makes a difference. If nothing you'll gain respect and some self confidence.
2. Dress Older! Look at the way you dress, the way you present yourself also makes a tremendous difference. If you dress like someone who's older, people will naturally assume you are older, even if you look young.
3. Act your age! I'm not saying to be too boring or anything, but the way you speak, the way you walk, the way you act, all plays a big role. If you come across as mature, people will naturally assume you are more mature. I was a project manager and had to lead people a lot older than me, but I'm confident in everything I do and speak with conviction, and it helped shed the young immature kid stereotype.
4. Finally with girls! Just get it over with! You're still young, there's no better time than now. First kiss, first bang, they're all not that big of a deal, once you get them over with, it'll be just like anything else. Some girls might be turned off by your baby face, but just act ****y and funny, show them you're not as harmless as you make look. And if that still doesn't work, there's plenty of girls who will still go for you. Once I got over that mental block, it just flowed naturally, so I suggest you get the initial stuff over with and just have fun and enjoy yourself.
Good luck man, hope it clicks for you. :rockon: