When it comes to relationships I think they can come in all different shapes and sizes and no two are the same. What works in one may not work in another, and I do my best to keep as open a mind as I can when seeing other peoples relationships. If two (or more than two) people manage to make something work for an extended period of time, I am glad for them.
In general though, I don't think age plays as big a role in a relationship as stages of life. In a relationship with between a 20 and 45 year old, I would wonder where each person is in their lives. If a 20 year old is still trying to find their way in the world while the 45 year old has established their place and knows what the rest of their life is going to be, I think it could be tough. If the 45 year old has made and learned from the mistakes that the 20 year still has ahead of them, it would make it tough. All people aren't the same at the same age though, which is why age in of itself isn't as important I don't think. I'm a pretty young 28 year old, I'd feel very young dating a 30 year with a house and nice job. I probably wouldn't feel as out of place around most 21 year olds though. Everyone is different I think.