I find myself mostly thinking as long as they are both of legal age and its consensual, then live and let live. Sometimes I'll do a double take coz society makes such a big deal out of it that its an involuntary reaction, but at the end of the day, whatever happiness that people can find in this miserable fucking world they should go for. As long as they arent hurting other people to get there (within reason, if a person has been an ass then by all means get yo vengeance )
I think it's all about who you want to be with. If the feeling is mutual then who is anyone else to stick their nose in and tell you what to do?
On the flip/reality side though, if you were the 20 year old, when you get to 40 then your partner would be 65.. there are clear life stages, differences and needs that are just fact and do impact on relationships.
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.