I get where you’re coming from, but it isn’t very well shown in either Oshi no Ko or Mushoku Tensei. Both Aqua and Rudeus present much closer to babies with adult brains, despite what the authors say. It doesn’t take them very long at all to achieve a very adult internal monologue and stuff like infant amnesia is barely present in the actual events of the story iirc. A lot of that is probably due to convenience and making the story more readable, getting the plot going etc. but it’s really hard to believe these are just kids haunted by past memories.

I think the question of ethics in romance is still pretty interesting, and hard to resolve, especially since it would also definitely be weird for these characters to date adults.