For me it has to be injury and the quest to avoid it.
I'm 40. Just a kid to some but old in the minds of a lot of fitness oriented people. I have learned that I can still do what I could do 20 years ago but I have to be much smarter about it. I can't just wake up and decide to attempt a PR in the deadlift or run a competitive 10K. It has to be something I build up to slowly. I also have to allow myself more time for recovery between workouts. And maybe the hardest thing to learn is when to skip workouts altogether. If I don't watch all of those points, I'm begging to get injured.
I have a friend who is 57 and in amazing shape. Very impressive guy. I once asked how he manages to do it and he said his biggest key has been to learn to "sell short" during his workouts. An investor can stay rich if he always sells his stock before it reaches its peak value. If he does not sell short and hangs on for the big payoff, he's risking a big loss. My friend applies the same philosophy to his workouts. He tries to always walk away from the bar or treadmill with a few reps or laps left in the tank. When I was a kid, I thought that was being soft. Now that I'm an adult, I think that's being smart!