I'm 46, so I guess I'm neither old nor young (actually feeling physically old while retaing my juvenile delinquent mentality!). Age isn't an issue for the most part. I've partied with people 1/3 my age and double my age and enjoyed being with them all. It's more about the individuals and the settings.
But I do have to admit (albeit ashamedly) that I am a bit wary of anyone younger than their mid 20's --- nothing personal, but they tend to run their mouths too much, which can be a VERY dangerous thing if you grow! And yes, I know that is a stereotype and there are absolutely people younger who I would fully trust, but I've got to really get to know them. Trust is earned, not given.
But, there IS something to be said for partying with a room full of very attractive girls in their early 20's --- even if they ARE my daughters friends!