While many people think "generational switchover" is because people are retiring, but it occurs in waves for multiple reasons just like project growths occur the same way. Signups for a lot of organizations occur in 'waves' as groups of people seem to appear at the same time. Maybe it was that they were influenced to join by some event or person, or maybe they all just graduated around the same time and were looking at for were similar. There are groups of people who joined Fedora from the 'Red Hat Linux' days, but there are groups who came in waves at different times either because a new technology was added, another distribution had some sort of split, or because their school used it and they found they liked contributing.

And just like people entering, people leave in waves also. Maybe its because they are leaving the workforce, but it is usually that people get married, have kids, changed jobs, got seriously ill and decided to change lifestyles, or just decide that moisture farming isn't as bad as Luke Skywalker thought it was. Most long life volunteer organizations seem to see regular turnover with groups coming in and leaving. Short lived ones usually don't last past their first or second generation wave because there isn't any want to bring in new people to rock the boat.