there was actually a big debate about this a month or two back with fujoshi_c and her new boyfriend, i think there was little more then 10 years difference in age...
there we go. the fakku community essentially agreed that although a difference in age isnt uncommon in relationships, there are times when it could be considered an extreme. i believe she was 18 and her gentleman suiter was 30, if that gives you an idea. personally i believe that if the relationship is genuine and the couple are content with the fact, who fucking cares? youll probably see differences in maturity, mentality... a bunch of things, but its nothing a good talk cant work out.