The age difference really isn't bit at all, it's what, 6 years? Most of my friends are dating guys 6 years older than them with no issue, the only reason it's weird is because we knew Dawn at 15/16 (but really, Xander dating Dawn, even at that age, is no worse than Angel dating Buffy at 15). As for the feeling part, I think Lyri is spot on, Dawn idolized Spike as the bad boy superstar, it was never gonna go anywhere, even she knew it wasn't real. And again, after the AR, any notion that Spike was cool went directly out the window. As for Dawn/Xander, I think the notion that there might be something deeper there started in s7. It wasn't romantic, but they each had a deep understanding of how the other felt. Xander/Dawn starting to develop at that point. I know, Xander was 22, Dawn was 17, but I think their relationship grew organically to a point where Xander no longer saw Dawn as the kid sister, but he saw her as a real, grown up, person. It wasn't about jumping into a relationship at that point, but I think the moment Xander stopped viewing Dawn as Buffy's sister, in s7, is the moment the idea of Dander became a reality.