I don't think age difference is that important. I know several marriages where people are the same age and they can't stand each other. They continue living this way because of the kids, finances and so on... Their age doesn't help them in any way... They just don't 'click'. There was an age difference between my parents (my father was 13 years older) but I don't think that was a problem. I guess you see things differently when you are 20 and differently when you are 40 but that doesn't mean you cannot build a steady and happy relationship. Yeah, people will talk and criticize but it's your life not theirs. They usually say those things to make themselves feel better about their own lives (which are far from perfect too, I believe). You only have to please yourself... Once you are happy the rest can go to hell.

Maybe I am wrong but that's how I see it. Good luck to you and your fiance!