And here's the opposite … sort of.
Here is a story - Older Athletes Push the Limits - from the LA Times that tells of older athletes still being able to push the boundaries of what they, and the rest of the world, think is possible as you age. It seems that the only thing that is certain is that - in the words of Hilary Walker - you shouldn't put limits on yourself.
There's no doubt that you will reach limits (is there?), but make sure you find them when you find them, don't decide in advance.