Anecdote, but this was the approximate case on the team I worked on at Google (Manager in his mid 30s, team mostly in their 20s with one in his 50s). It happens. Actually, it was nice to see, as some evidence of non-ageism.

I hope to no longer need to work by the time I'm 50, but nobody knows what the future will bring, so I'd certainly like to have the option just in case!