Can someone please explain the vesting of retirement travel benefits again? I know there is a formula used, but not sure I understood it. Asked differently, what would be the latest you could join United before age 65 and still get vested?

Also, those bennies are not guaranteed in the CBA, correct? So, theoretically, United could change those or the privileges (boarding priorities etc.) anytime, like they did for the Flight Attendants couple years back (I think it was United, retirees with >20 yrs used to have better boarding privileges than active employees).
Thanks!