Unless you're talking about the truly elderly, that's not true. The average age of an entrepreneur in the US is about 38. (The average age of successful entrepreneurs is even older) It skews younger in tech, but the middle aged are still more likely to start a business. After all, they tend to have more experience, contacts, and money.
I would say that has everything to do with demographics. Older people are in general less entrepreneurial due to more risk aversion.