That's a lot of questions... I should really learn how to use quotes, but I'm lazy.
So, MC is about 14-15;
there's essentially no difference with what you get from old / young animals, unless they're young enough to have embryonic instead of adult stem cells (but even then there's basically no difference in effect due to quirk bullshit);
Shima is the surname;
and Izuku doesn't think MC qualifies as female, so has no difficulty talking to them.
I hope this makes things clearer, but knowing my communication skills, probably not.