I'm going to side with Jimbo. My god, as a 20 year old i was a complete nutcase and though I had travelled a lot and already lived more than most 40 year olds I can't see what I would have had to offer a 40 or 50 year old that a 30 year old couldn't have bettered, save for a perter bottom etc.
There are always going to be exceptions to every rule but generally if a 20 year old is going out with a 50 year old it's not going to be for the same reasons two 30 year olds would hook up for. Most will be looking for a better or different life. Some will be looking for daddy.