Is there any data available on the numbers of people starting companies in each age group? Perhaps older people really are less likely to try it, which would explain why there might be fewer successful startups founded by older people. Without that number (startups by age), it is impossible to deduce the likelihood of success by age just from the number of successful startups by age.
Without seeing such numbers, it seems plausible to me that there would be less older founders for reasons that have been cited before on YC. People with families are simply less likely to take the risk.