My husband is three years younger than me and my sister's husband is six years younger than her, but this works for us. I have many friends in relationships the other way. One of my friends is 15 years younger than her spouse. To me, this seems a big ergy, but I'm know I've had people look oddly on my own relationship. I think it all comes down what works for different people. Its a huge place out there and thankfully, we are not all alike and therefore all relationships (should) be different!
While love can hold age different relationships together most of us watching someone really young with someone really old as being about money. I think that's the general consensus because the majority of people will seek after someone around their own age.
My view on the matter is that as long as both are legal adults with reasonable maturity, age should not matter when it comes to relationships. Even if there is a ten year difference as long as neither is underage, a relationship could work.
I'm not going to put a specific age but good sense should prevail there is enough heartache in relationships so you don't want something like age to be an additional factor. If you can't handle it don't go in.