What is this advice you people are giving? Tell her the truth? What kind of gay shit is that. Lie brotha, lie. If you want to get serious about this girl you should probably tell her the truth at once, but hey how serious could you get when youre sixteen right?
You have to be careful when lying though, all the situations that might come up and catch you in a lie has to be minimised. For instance you have to avoid places where you might have to show id. Like buying smokes, going to the club, etc. Also you have to keep her away from you friends and family because if they ask her how old she is the distance in years are bound to come up, So its just too risky.
Youre friends could be cool though, i suspect you have mostly guy friends right? They could easily lie for you, if they are real friends that is so your friends shouldnt be a problem, what you have to watch out for though is those bitchass friends that are jelous of your situatuion and could put sticks in your wheels. So its always a risk involving your friends.
About your family, i doubt you could make them lie for you so the best way would to keep them out of the picture completely. The problem is though that after some time she is going to want to meet your family and wonder why you never let her meet them. She is going to think you are ashamed of her and that is going to give you massive headaches. The best way would be to fabricate a story as soon as possible that your family is horrible and you are afraid of letting her meet them. That could work, but if she is the nagging type you would probably have to squeeze out a few tears to convince her. Tears are always good because they will give you undurstanding and sympathy.
There you have it, its not easy but im sure you are up for the challenge young blood. Good luck