>>149004275
>She was also acting much older in those issues

No, she wasn't. In her first appearance she looked maybe college aged, but never talked about going to college or anything. She was a girl from the FEAST Shelter that Spider-Boy saved, and her parents told her to keep an eye on Bailey. Two months later she was appearing in Spider-Boy, and was pretty obviously in his age group - a couple years older, but in the "7th grader hanging with a cool 5th grader" sort of way. There was nothing in the writing of those early issues to suggest she was, for example, a college student looking out for a 10 year old. She enjoyed going bowling with him as another kid going bowling, not teenager looking after a grade schooler.
It's certainly possible that Slott completely rewrote the story between her first and second appearances, which we separated by only a couple months. It seems substantially more likely that Slott's script said something like "Christina Xu: A girl a little older than Spider-Boy," Ramos made her a LOT older, and nobody thought it was worth the time and trouble of changing the art.
I could be wrong. But I'm pretty sure she was always intended to be his slightly-older peer, and we just got some artistic crossed wires.