@Bramia: Believe me, dude, I get where you're coming from. I thought that too at first, but let's remember a few things: He literally had to redo his birth with his memories intact. If it was just a matter of "getting a new body" like other isekai, then yeah you'd have an argument. But he literally had to restart life.
Also, this whatever-God took pity on him and let him keep his memories, but still checks in from time to time.
And I know where you're coming from: He ACTS like a kid from time to time, but he was also deprived of a childhood in his original life (something I can personally relate to). He got a second chance to not only be a kid again, but used his knowledge to keep his family from falling apart. Over time, he actually assimilated to being an ACTUAL kid due to his experiences, hence his big breakdown scene when he cried.
Keep in mind that he was also trying to obey the morality laws of this world where age 15 is the age of adulthood. If anyone is preying on the young, it was her; not the other way around. He gave in out of love, not lust. He had plenty of opportunities before this but never budged.
Fact of the matter is, Rudeus IS a kid. Going past that is overthinking it. Not knocking you for it, though, I'm guilty of it too, man. Everything about this world has made it clear we gotta look past conventional logic and bend it to understanding them logic of THAT world, not ours. If you can't do that, maybe it's time to consider reading something else? I have a feeling it'll only get worse from here.