The only thing I notice is that I don't move as quickly as I used to. It just takes me a little longer to do what I want, but I sure haven't given much up. I don't drink like once did, and don't dance all night (I'm through after a few hours), but like Dave said, it is mainly mental.
I sure ain't as good as I once was.
I have to agree with Tim, the best ship is the next one I'm going on. I have seen every age range on the Carnival ships. To get a little technical though, the Spirit Class ships are the best in my opinion.