In a recent episode of the television show Entourage, Ari Gold (a 40 year old man) says:
I've known her since I'm 19.
In an episode of Sex and the City, a character, who is 15, tells Carrie:
I've been giving blowjobs since I'm 12.
The speakers are replacing the past tense of the verb to be (was) with the present tense (am).
Is this simply a mistake of the writers (seemingly specific to HBO), or is this usage a trend that is developing? I've heard it in other television shows and in person, as well.