BuddyPress.org

@randygootjes
This would be easy using just BuddyPress Xprofile fields:
https://codex.buddypress.org/user/buddypress-components-and-features/extended-profiles/
You could create a dropdown menu and make it required and only allow the selection from say 21 to 120 years of age (“Not like most of us will live that long anyway unfortunately”) But also have a check box that is required that all information entered to join the site is true etc.