This Tree Gains Another Ring [Birthday Train]

My poor eyes..

International drinking age [eg, what applies when over international waters] is 18, and that seems to be the most common application (though many places allow you to drink at 16- eg, France, Germany).

https://en.wikipedia.org/wiki/Legal_drinking_age
The violet and red portions of the map on that page are places with >18 drinking ages. If you look at just the dark red for 21+ and 25+, America does stand out as a truly rare aberration in its restrictiveness.

It's not illegal unless the picks are in a sexual context, but peoples standards for that vary greatly.

It's not illegal for medical, scientific, artistic, religious, personal [eg, charting growth or nudist photos]/etc purposes, so long as no exploitation of the child or sexually-related distribution take place. [Note, child abuse laws also protect here.]
More specifically, child pornography legally only covers images relating to "sexually explicit conduct". Any non-sexual/non-exploitative photos would be legal.
In extension of that, selfies which don't have a clearly sexualized presentation would actually be fully legal.

. ..well, that's the concept, and how it ought work in theory.
In practice, basically any sort of transgression is risking legal action, namely due to puritan standards toward what is 'sexually explicit conduct' (tl;dr, anything) and a desire to punish kids for being sexually active.

similarly, any art of a fictional minor engaged in sexually explicit conduct would earn you up to 5 years in prison. Or rather, any art of characters "who appear to be under 18". Considering how many westerners [who are not particularly familiar with anime or Japanese stylization] think all anime/manga characters look underage, and the fact that the same "if those in power say it is something, then that's what it is" rule above applies here..

Well, this article is actually why I'm familiar with that nuance- and while it may or may not have been a misinterpretation of age in that circumstance, it did lead to a lot of realizations floating around about how much free reign there is for those enforcing the topic to define things as they see fit.

Interestingly, this all theoretically applies even if you drew the art yourself- for example, david Willis does sexual sidestories for his college webcomic.
Currently, if anyone considers the characters to "look underage," or if Willis himself says one is "only 17", then it'd be a violation of the current implementation of the law [and distribution charges, as well, presumably.]

Of course, going after a successful and well-known author, linked deeply into the convention community..
The legal status of censoring art without secondary basis is.. well, the one person sentenced solely off owning sexualized art got a 6 month plea deal with no further repercussions, because prosecution was worried about the constitutionality of the matter :P
There may be a reason (other than a lack of paying any mind to webcomics) that they don't attempt to take on someone with the actual ability to fight back. [Much like how the RIAA enjoyed picking on teens and preteens, I suppose.]

so in that sense the "I say it's underage, so it is" may become irrelevant in time, as soon as the free speech angle is pushed.
As far as sexting, however, I haven't heard of any progression toward formalizing what is and isn't "sexually explicit conduct," or to make sure the laws are changed to only punish those exploiting or abusing/etc minors, rather than dramatically punishing any teen that whimsically wants to show off their body to someone else.

tl;dr Ignoring any considerations on the topic matter itself, or views of how people "ought" to behave in regards to their own body or artistic expression, current implementation does not match the spirit of the law, or even constitutional expectations
As such any sort of nude photo of a minor or sexual art where the involved characters are not clearly adults will likely cause you problems to possess, regardless of context.