Technically, girls hit puberty earlier than guys, most on average have hit it by 12. Now, it's still messed up. Still way too soon, just, physically, yea, average girl will have hit puberty by then.
Gotta remember, average were having kids by 13/14/15 back in the distant past (hell, gross at it is, youngest ever mother was ~5 years old, yep....).
Anyways, yea, age gap wouldn't bother me if she was 20 and he was 35 lol. But being 12 and 27? Eww.