To be clear, I mean that the the way US laws are enforced, to the best of my knowledge, being underage yourself doesn't mean you are off the hook for having underage material.

I'd be interested to learn if this isn't the case.

What if I'm 30 and have sexual images of my own teenage self that I myself took in the mirror, say, 15 years ago? This just seems insane

Yeah, to the best of my knowledge.