Licorice Pizza is maybe my favorite PTA film. To me he is one of those directors who pits some of the greatest actors together to wring out those great tortured performances, but all for nothing, for some uninteresting story (There will be Blood, The Master, Phantom Thread). Or maybe I just don't get it. That's why I liked the lighter ones better, like Licorice Pizza, Inherent Vice, Boogie Nights.
It's ironic how nowadays they have to squeeze into a two-hour film what could unfold over the course of a TV series season. This story could definitely have been a one-season series to flesh out all the nooks and crannies we know got a drive-by. But maybe much of the poignancy has to do with working the story into the limitations of a film - something you have to watch in one sitting.
I was a teen in the 1970s and we were aware that 18 was legal consent. But of course different states different statutes, and I don't know what California law was at the time. Maybe it was 14, or maybe it depended on the age difference. A 15 old and an 18 might not trigger the law, but ten years is a big difference. However if you mean that laws like this while on the books, weren't strictly enforced then I'd agree, especially when the adult was the female. But if the minor's parents went to the police then that would change the dynamics of the situation.