Sort of irrelevent for the OP, as the relationship in question is a homosexual relationship, and there aren't exactly a big group of studies to display what kind of mates are generally picked, but males tend to go for younger females for a variety of reasons. In this day and age though, it seems that is being defied to a pretty large extent. Hard to say why society is kind of pushing around like that, but regardless.
The relationship in question, I do find a little weird. On the other hand, my 17 year old friend was dating a 21 or 22 year old (both female), and common ground was easily developed there. If a relationship of that type can be developed and held in a healthy way, I see no real problems with it. We are just sort of trained to believe that a relationship between certain age groups is wrong.
Personally, I aim for slightly younger and don't find myself attracted physically or emotionally to nearly any girls my age (I'm 17 personally). I found one who doesn't even go to the same school as me, and that's it. I mean, there are still those that are physically attractive, but at the very least, I've learned that isn't nearly enough for myself. I guess I sort of fall into the trend, though I would never consider going for somebody past two years younger than me. The only girl that I say I've gone head over heels for in the past couple of years is a year younger than me. Older than me though, I'm not sure. I think I would really have to experience a relationship such as that to really say if I think it is potentially workable.
I feel like I just kind of rambled.