My understanding is that much of the YC evaluation comes from accomplishment density - accomplishments/time. That's pretty hard to maintain for many years, so it's natural that fewer 40 somethings meet that metric. I'm 44 and can easily chart my regression-to-the-mean over the years.
In fact, I think if somebody does maintain that accomplishment density and is interested in YC, they become a partner. :)
Yet others in this thread have explained low numbers of over 40 employees by claiming they often go on to become entrepreneurs. In that case we should see a higher precentage of founders in that older age group not a lower number.
Of course what you and others say is quite plausible. But also often contradictory. It really looks like a very real phenomenon for which intuitive but post hoc conjectures are being generated. You could do the same thing for women or any other group as well (thank goodness sensible people don't)
Ycombinator is not evil or unusual but really ageism is a very measurable, irrational problem in the industry that is undoubtedly resulting in lost efficiency and we have a choice of addressing it or just ignoring it with facile explanations.
In fact, I think if somebody does maintain that accomplishment density and is interested in YC, they become a partner. :)