What is the age gap ??

Probably 10 or 12 yrs. Could be 8 too who knows.

That's a generation gap

But nothing too uncommon. I'm used to it now. If both of them are adults then it's their choice. Although they met when ml was just a child and ml might have had a crush on mc, but I'm sure mc didn't have any questionable feelings toward a child. Not until that kid appeared as a hot, adult man and started actively chasing after mc.