I was thinking something similar. Not knowing anything about the actual applicant demographics, my intuition is that they would skew younger.

From personal experience, the overlap of the sets:

1) of people over 30

2) of people who are entrepreneurial

3) of people who have actionable ideas

4) of people who are in a position to take income risks

5) of people who need the money/PR/connections that YC provides

is pretty small. There just aren't that many 30-something entrepreneurs who are in a position to need what YC provides, and who are able and willing to take a vacation from what is probably a substantial, stable income.