Proof of age can be verified in many ways, you can get an ID card from your state. You can also use a birth certificate, passport, or any other document produced by a legitimate state or federal agency.
If you do not have a drivers license, you can use your birth certificate as proof as to how old you are. You can also get a state id which will show your age as well.
Certified copies of birth certificates; birth record cards issued by some states; non-driver identification cards issued by most, if not all, states; passport books; passport cards; and military-identification cards are about all there are.
The best proof is an official, certified copy of your birth certificate. You may also want to consider getting an official state identification card. These are generally issued by your state's department of motor vehicles, and do not require that you know how to drive (or permit you to do so).