Re: I'm 18 and have been having sex with a minor since I was 17
To your first question [Am I a kiddie- . . now that I am 18?] Yes and no. Yes, you are; but not just because you turned 18. (I'm only half-joking.)
Penal Code §288 §261.5 and §647.6 proscribe (ban) sex with a minor and sexual touching of a minor, respectively. (I am giving this answer from memory, so I will need to augment it with specifics once I look up the statutes in terms of punishment, so this will be a 2-part answer.)
The statutes can be violated by a minor-- all that means is that the minor MIGHT be treated as a juvenile and dealt with [ostensibly less harshly] in juvenile court. So it's not that when you turn 18, as is often the misunderstanding, you are then and only then committing a crime. It was a crime when you were 17 (or 16 or 12).
The law (particularly PC§288) punishes more harshly acts committed on a minor who is more than a certain number of years younger than the perpetrator. You are not within that group at a difference of 2 1/2 years.
In terms of punishment and the actual elements of each crime, you can look them up on the internet, or just wait for part 2 of my response. However, PC 288 (child molestation - a felony) and 647.6 (and probably the statutory rape §261.5) require lifetime registration pursuant to PC§ 290, which means each year on your birthday you must go to the sheriff's dept. and have them take your picture so they can add you and all your info (address,etc.) to the list of child molesters in the neighborhood, which list is open to the public every single day (and in most states now available on the internet, sometimes with a link to mapquest to find the exact location of your house.
The more salient point is that a police agency or prosecutor would have to be aware of the crime(s) before you would be prosecuted, and, as a practical matter, someone would have to care. If, for example, the two of you are living in her parents' house and they are -sorry- stupid enough to encourage a sexual relationship between teens, you likely don't have too much to worry about except pregnancy, STD's or death from AIDS-- unless you piss off your girlfriend or her parents, of course. Ahh, young love. . .
I'll get back to you with the rest. Good luck. (If you wish, you can call me directly at 800-515-0233 or email me at jgoodmanrubio@hotmail.com. Peace.