IANAL, but... Images of simple nudity, regardless of age or gender, is not illegal in the US. Numerous cases have been brought, mostly as a result of someone at a pharmacy photo shop reporting something, but no cases have withstood appeal. Issues that have come up include parents taking pics of their kids in the bathtub and photographic or drawn artwork such as by Sally Mann. A friend of mine who lived part of the year in Barcelona was reported because pics he'd taken of his wife at the beach (she was in a bathing suit) included nude kids in the background. The case pretty much ruined his life for over 2 years.

To be illegal the images must include an element of lewdness. I don't think the TSA images would qualify as generally illegal.

HOWEVER, coercion is illegal. If a photographer coerces a 16-year-old to pose nude 'or else' then you have a criminal case. The 'or else' could be any number of things from 'I'll hit you' to 'I won't let you go home'. If there is no coercion and the photos are taken with the full and willing permission of the subject (and in some jurisdictions also that of their parents) then there are no legal issues.

SO, one question then is if the TSA are effectively coercive in saying that if you don't pose for a nude image of yourself then 'I won't let you go home'.