IMO the gulf of maturity between someone who just learned how to drive & someone who has been out on their own for at least 2 yrs is huge. Fine, you can have sex - but chances are she's going to be looking for a relationship. You want to go to "the dance" on Friday nite w/ all the other teenieboppers? What sort of things do you talk about? All her friends on the cheerleading squad who are bitches?
Sure it can depend on the particular girl, but I've generally found that people who are at different places in their lives (doesn't always mean an age gap) dont' make for good relationships. And she's probably emotionally very immature (LMAO... not that guys aren't at age 20 or 40... but guys aren't generally as cerebral about it as girls are).
Also wonder what her parents say? Since she's under age she is still the responsibility of her parents and you aren't -- what do they say about it?
Or are you just looking for a 1-niter?