Early, Fox? Well... I guess everyone matures at different ages. Boys too.
He was 10 when Ayako adopted Miu, at the time of the bath some time has already passed, so let's say 10-11. Yeah... Not early from where I'm sitting XD
I can pretty much guarantee I was already looking at girls by that age.
Anyway. On a more serious note, and leaving aside the Mama-Shota goodness, memes and jokes. That attitude from Ayako bothers me. I've seen it here, in other manga AND in real life too.
People would frown upon a grown man pulling the panties of a 10yo girl, let alone getting in the bath with her (I'm gonna say in Japan too, tho not sure). But it's like boys don't have the same rights.
It's like we don't have feelings. So what? Just because we're 10 we can't feel embarrassed at a woman seeing us naked? Do we not have the right to a little privacy?