I think phased retirement is the best approach. We see this in academia all the time and also at NASA. There’s a period of time over several years where people start doing part time work, then even after retirement they’re often brought in for consulting. At NASA in particular, it’s been 50 years since Apollo and NASA is trying to go back to the Moon and these old folk have tons of irreplaceable insight. Invaluable, really, and in my opinion their limited lifespan adds (or ought to add) a significant sense of urgency to NASA’s current efforts.