How is Return to the Blue Lagoon legal if Milla Jovovich was 15 when it was shot?

It was not unlawful. First of all, child pornography is illegal, but child nudity is not. A nudist family may have images of their children, nude, and it is not considered pornography nor is in in violation of laws in the U.S., at least. This is part and parcel of the issue surrounding the definition of pornography and the "I know it when I see it" decision-making in the judiciary.

In the end, though, pornography is generally construed as having explicit sexual content. In fact, an underage person can be clothed and involved in sexual acts and it would be child pornography (and thus unlawful). An underage person could be nude and not in a sexual situation or position and this would be child nudity (and thus lawful).

I hope this helps. The actual laws surrounding this topic come from a variety of codified laws in the U.S., primarily federal but also state and local, so your mileage may vary state to state and city to city. But this is the general rule.

Nudity is not, in itself, pornography.

If only one person is present in a scene (a photo, a film set, whatever) you ordinarily do not have pornography, as it generally requires the presence of two or more participants.

This was a rule of thumb in 1970s British cinema, which normally didn't object to female nudity on screen, so long as there was no man present alongside her. This seems to have been agreed with the British Board of Film Censors, on the basis that a female body (unlike a male) is incapable of showing sexual arousal - at least on a cinema screen.

UK newspapers used to publish topless photos of female models ("Page 3 girls"), from about 1974 onwards, and it was generally regarded as merely "glamour" photography.

There was, in the 1950s, an unhelpful "I know it when I see it" attitude. But that was wholly subjective, hence useless.

After the introduction of the Obscene Publications Act 1959, in the UK, nudity ceased to be pornography in law, if it ever had been ("It is evident that mere nudity in painting and sculpture is not obscenity": Buchanan's Journal of Man, published in January 1888). Pornography was not even the issue: a publication now had to be 'obscene' in order to transgress the law.

That meant it had to "tend to deprave and corrupt" the intended audience. But in 1963 the Act's impact was largely destroyed by the Lady Chatterley trial at the Old Bailey, when Penguin Books were acquitted by the jury on a charge of having published the (fairly sexually explicit) novel, Lady Chatterley's Lover, publication of which had been banned in the UK since the 1920s.

This decision pretty much drove a coach-and-horses through the 1959 Act, and in the wake of the trial something had to be pretty strong indeed to be in danger of being judged 'obscene' by a jury, mere sex (between consenting adults), or nudity, would not do it.

By criminalising it, the 1959 Act had surrendered the decision to a jury. No longer could a judge or magistrate impose his view of what was obscene. In order to succeed in a prosecution, you had to convince a jury. That was quickly seen to be quite hard to do!