Underage girls are under the age of consent for sex (and some other things). In a sense they are still “owned” by their parent(s) or legal guardian(s). Legally they are not allowed to do it and we’re not allowed to do them or even to be getting there with them. It’s all strictly prohibited and there is a great stigma attached to any such relationship or even to any hint of one.
One such hint might be the act of objectifying such a girl.
If you indicate to another person that such a girl has a nice butt then that could be taken as a hint that you might like to tap that. Even if the other person doesn’t judge you, you are still in the world of stigma in a broader cultural sense.
Perhaps we could class such a hint as an indiscretion but there other types of hints that might be more plain in their implications. For example: Showing another person photos of underage girls who while not being abused or legally misused are still pictured with the purpose of arousing admiration for their beauty. A person then could be forgiven for wondering if that’s your “type”, if you prefer underaged girls to adult girls.
This isn’t my last word on the matter. There are other things about this topic that I want to deal with seperately but for now I just want to broach one last matter. There is a downward sliding scale of nasty in the photography of underage girls that beyond a certain point, I feel is harmful for all involved. Without breaking the law one can still feel wretched for looking at certain photos. There is a genre of photography in Japan called U-15 where some of the works are so low on artistic merit and so high in their lasciviousness that one can taste already what it might be like if they were to take the next step into the downright illegal.