Yeah, the fact that she was IN what's technically child porn was the joke, but from what I remember the point was that the movie Hank was accused of renting was the one where she was legal.
The actress with the tattoo was a different actress. Here's a quote from it I just copypasted:>Sixty-eight minutes into the film, actress Dee Dee Cupp bends over to shoe her horse. Now, if you pause the tape and look closely, you can just make out a tattoo on Dee Dee's left buttock that says "I heart Charlie Sheen." Mrs. Cupp also makes an uncredited cameo in Jail Bait, a tape which marks veteran porn star Fernanda Valli's return to the adult film industry on the occasion of her eighteenth birthday. At the seventy minute mark, Dee Dee bends over to shoe a donkey. What do we see on her left buttock? Nothing! There is no tattoo! This can only mean that Jail Bait was made before Cuffs and Collars.
Fernanda Valli is the jailbait actress. Dee Dee Cupp is the one with the tattoo and doesn't have a specified age.
I mean, either way Hank probably knows more about jailbait porn than the average person; we just don't know for sure if he was actually watching any and the film he "rented" apparently wasn't.