Joe said
In the 60s and 70s I think things were a lot more relaxed with underage nudity (eg Jenny Agutter's naked swim in Walkabout).
Ever see the 1971 movie Friends? (Not the TV show of the same name.) IIRC, it's about this teenage couple that runs off to Paris and they, of course, fall in love and consummate the relationship. The two lead actors were both right around 16 and, much like Jenny Agutter, showed off their wares. The '70s were definitely a lot more liberating in terms of kids not quite 18 (but older than 15, ...