It's true that there seems to be a sort of cliff around my age; there are very few people in product development roles over the age of 50. I would attribute that to the way the industry evolved: mine was the first generation that really had the opportunity to grow up with PCs. My first exposure was a programming class in summer school in 1981. The demand for software developers also really exploded in the 1980s. These things combined to make software both more interesting and more compelling as a career than it had been in earlier generations.
EDIT: It's probably also worth pointing out that anyone who'd joined Microsoft before ~1995 and was careful about managing their money could likely have decided to retire by now.