I don't think this is about a legal wrong but more about a moral wrong. If he were 18, for instance, he'd still be legal, but many people (including herself, presumably) would look at it as morally wrong for a woman in her forties to date an 18-year-old boy - which is essentially what he is: too young to go to bars or drink (in the US at least), for instance.
Edit: hence I don't agree with #2's version, either.
Edit II: this reminds me of myself when I was 19, living in New Orleans, and dating a woman in her forties. But at least the legal drinking age in New Orleans at the time was 19, so I just barely made the cut. :-)
Edit III: maybe "jung genug um gerade nicht mehr (gesellschaftlich) tabu zu sein"?