I've read that in medieval times, an older husband was considered a good match as he would have someone to care for him when he aged, and was likely to leave the woman a well-set-up widow. However nowadays large age differences seem to be looked at with a mixture of amusement and suspicion. When and why did this U-turn come about? (I'm thinking specifically of western marriages where the husband is older; I think older-female marriages have always been scorned, although if anyone can give me any insight on that or views in other parts of the world, I'm very interested to hear it too).
You'll need to be more specific about time and place. Also, remember that the medieval weddings we have on record were for the upper classes, for whom marriage was a political decision rather than a personal one. This part of the FAQ might be helpful to you.
I'll be doing some of my own research on this topic. I've taken a few medieval history classes, but I don't remember enough specifics to feel qualified answering this question myself.