This answer is late so idk if anyone will read it, but when I was 12 there was a 17 year old guy who I had something going on with. From that experience I know why it's wrong. Now that I'm older it pisses me off that everyone just sat back and let it happen. I was just 12, me and him weren't at the same stage of development (mentally or physically) and it wasn't fair for me. He had the advantage and I didn't, I didn't know about that type of thing, and really I'm 15 now and if someone like him wanted to be with me now I would say no. The problem is even if you are old enough to enjoy it you still aren't old enough to consent because I was too young to know that he was just taking advantage of how young I was and how easy it would be, along with that I was too young to know he didn't care about me at all. Plus lots of 12 year old don't know how to respond in sexual situations and will do nothing to stop something they know they don't want but don't know how to stop.