It's quite funny, but most teenagers are walking bags of raging hormones so it doesn't take much to get them excited. Although when I was a teenager at school (1989-94) we didn't have the world wide web - I'd have thought most teens nowadays would be pretty indifferent to the sight of bare breasts, given the amount of nudity available online.
Nowadays we wouldn't allow a 16-year-old to get naked on camera - it'd be illegal below 18 here in the UK. In the 60s and 70s I think things were a lot more relaxed with underage nudity (eg Jenny Agutter's ...