Happy Birthday Peter.
Btw, the whole rules for sexual consent is more confusing then just being under or over 18 years of age.
It also takes into account the gap of years between the two people in many states.
I think in CA you're allowed a 2 age gap, or something like that. So if you're 18 and your gf is 17, then it's fair game. Im not positive its like that in CA, but I know it is in several states.
Edit: Scratch that. In CA it works as so: Sexual encounters between an adult and minor that have no more than 3 years in age difference is only a misdemeanor. Getting a misdemeanor doesn't mean much.
While 2 years or less only leads to a civil penalty (pretty much, a fine).
So you won't get arrested, but you'll hafto pay stuff.