Others have said it, as you grow old you can go harder on a given day than when you were younger. I'm specifically referring to running and training for endurance racing, but I suppose this would also refer to anything from studying to negotiating.
Tuesday (yesterday) I ran a workout with four different groups during track practice. I ran with each group until they were either injured or done. When I finished there was no one left running. To go from my moderate state of fitness to doing such a long and fast workout was surprising. I ran a 47s 300 m, 66s 400 m, and 14 s 100m, among about three miles of hard running. Yet toward the end, when I wanted to quit, I knew I could keep going, because I've been to that point in workouts and races so many times I know I can keep going. It was also the oldest I have ever been when running a workout.
I suppose this is just a perk of getting older. Most of the best ultrarunners are much older than the best track and road racing runners, in part because I suppose they benefit from knowing they can keep going when the going gets tough.
The downside is I don't seem to recover as fast as when I was younger. Although recovery depends on so many things, I don't think that is a hard and fast rule so much as maybe a consequence of doing harder workouts or spending less time on the little things that improve recovery. (At least that's my rationalization.)
2014 is going to be a good year!