I think Anya and Xander take the cake for the age difference even though Xander was 18 and legal the first time they hooked up. They have the biggest age difference out of any of the main couples on this show and that isn't an easy contest to win.
Their age difference probably plays more of a factor into their relationship than the age difference between Xander and Dawn unless Xander really did grow up towards the end of season 7(which is a definite possibility). Their age difference was so big that it completely changed their outlooks on life. For Xander, he was young and confused and for Anya, she was at the end of her life and wanted to settle down. This was their fundamental problem. Anya felt like she was at the end of her life and Xander was just accepting adulthood.
Of course, now Xander is grown up finally and Dawn may not be there, so you may run into that problem again but with Xander on the other end.
My problem with Xander/Dawn isn't the age difference. I just don't like it romantic. It was a good familial relationship. I don't hate Xander/Dawn, but I just don't ship it. It doesn't jump at me as something I want to see work.