Then your rebuttal was reductio ad absurdum. You cannot discount that a huge amount of maturing occurs in the first couple of years. That is why it is acceptable for people to start "going out" during highschool; it's the accepted line from which people generally start.
Nonetheless I do see where you're coming from, but nonetheless the age difference here is unimportant. She shares morals which are akin to my own, and in terms of maturity I find we're much on the same level, at least for most things such as social interactions and intimacy. We know each other's limits and respect them, and for that reason I find the age difference to be ambiguous or arbitrary at best.
In Annabelle's case she's December of '96, meaning she's been thrown in to a group of Year 10s (Sophomores I think in American terms), most of which are a year older than her, and therefore her social maturity has been forced to conform to that anyway. In essence, two school years separate us, and that's acceptable by most standards.