Just to explain my math point a bit more, let's take the definition of recent by decade, where all presidents serving within those decades count:

• 1 decade (2014): 3
• 2 decades (2004): 4
• 3 decades (1994): 5
• 4 decades (1984): 7
• 5 decades (1974): 10
• 6 decades (1964): 11
• 7 decades (1955): 13
• 8 decades (1945): 15

Even going back fairly far, we still have a pretty small sample size to draw conclusions for presidents specifically.

I agree with you on the age issue as a broader problem. There we have a solid sample. We've become a gerontocracy at the federal level especially, with the older generations holding onto power far past when they should have moved aside to allow in new people and fresh ideas. People in their 80's and 90's holding on to seats clogs the pipelines so that everyone else is prevented from moving up.