I think what Andrew is trying to say, is your age difference is - You don't normally find a 15 year old girl dating a 20 year old guy. Not that there's anything wrong with it, but you just don't usually see it. And sometimes when you do hear of that, relationships, they don't last very long because of the ages, and because if the guy is 5 or so years older than the girl, and especially if they're both that young, the guy isn't going to be as responsible. That's not always the case though, so don't think I'm being stereotypical or anything.
Plus, if you all love each other - That's really good, so. And Andrew's just trying to forewarn you so you're not hurt (not that he's sure you will be).