The countless hours of voluntary unpaid work by many people shows don’t act only for their own benefit
It’s true that first you care for yourself but after survival is secured many people care for others without profit concerns.
But if you need more time to satisfy your own needs you have less time for others.
More concrete data are hard to find, AmeriCorps' only started in '02. We can look at volunteer rates across demographics[1] but it's likely true that volunteer hours drop off with age.
[0] see: Bowling Alone (needs a post-COVID update, though)
[1] https://americorps.gov/sites/default/files/document/Voluntee...