Obviously when we're younger our reflexes are better and we're physically capable of doing things quicker than when we get old. The flipside is that as we age, assuming we ride often and keep learning, our experience, knowledge and wisdom increase.
Here's a blog about the aging motorcyclist.
Five Hundred Miles...: April 29, 2007
Not a fun prospect to think about, but certainly better than the alternative.
I tend to think that professional racers are usually at the top of their game in their late 20's. That's when they're still young enough to have great reflexes and physical ability, but they're old enough to have had a lot of experience, knowledge and a bit of wisdom.
For street riding, tho, the physical demands are not as great. I have to think that the average street rider probably gets to be a better rider until his late 50's. What do you think?