60+ become candidates because of accumulated wealth, accumulated critical mass of professional and political connections, and a bit of end-of-life ego.

As for what old STEM people do, my father started to learn C++ at 66, learned Python at 68, modified and invented variations of heat pipes for energy efficient home cooling, designed and built a home-brew control system (before weenie arduino stuff), mastered the art of the perfect grilled pork burrito, and learned to play jazz clarinet. Really miss that cranky old geezer. RIP, Dad.