Even if you persist a timestamp like 'birth date' into a database it doesn't matter - you can still cache the resulting 'age' calculations as CRDTs. But yes, it is a made up example. Another might be 'first/last observed time' for an IP address.
It used to be between 21 in most of the US until the voting age was lowered to 18 in 1971. It was then either 18 or 19 in most states (generally the more liberal ones) until 1984 when the national minimum was passed. 19 really is sort of a sweet spot for socially liberal North America.
Seriously, want to make some easy money? Build this.