Is a large age difference an issue in a relationship? Of course it is. It's not insurmountable, however. You just have to recognize it and deal with it honestly.
My husband is just shy of 10 years older than I am, and while I agree that the attitude that "age is just a number" can be thrown about in deference to the heart being blind to such matters, there are certain considerations one has to make with even a 10 year age difference in a relationship.
On a daily basis, it's a non-issue (aside from the good natured ribbing that is abundant at our house--"Honey, when you graduated from high school, I was in third grade!" or "The year you lost your virginity, I was six years old!"). I fell in love with him, it would have mattered very little to me how old he was. I suppose that it matters when you meet someone, as well. There's a period of in most of our lives where age means much less than it does when we're very young or getting 'old'. I was 23, he was 32. At that point in our lives we were pretty much peers. It would have been a different (and creepy!) story had we met when I was 15 and he was going on 25.
That is not to say, however, that any significant age difference doesn't require a little serious forethought. Planning for a synchronized retirement makes for some pretty creative fiscal management and investment strategies. Planning our family took a some consideration about the future. We have two children and have decided not to have any more (though we both wouldn't mind having another) with our age difference as a major factor in the decision. And even our 10 year spread makes for some fear about living out our golden years together. Fortunately, my husband has promised that he will outlive me, and insists that I'm lucky to have found him because a younger man could never handle me...