When does a life jacket needs replacing?

[Refueler]

You completely miss the point.

Folks are trying to assess if the performance standard has deteriorated over time. They are not assuming the straps are new.

Lifejacket straps are not made like seatbelts.

LJ straps do have to survive the initial inflation - where the action will increase the tightness of the straps - plus the sheer bouyancy hitting the 'system' in a matter of secs will be serious stress.
LJ straps as I have are less width but same webbing construction as seat belts ... as I understand it - seat belts are wider because they need to be comfortable to wear and also wide enough to distribute the force when in a crash ...
I disagree that LJ straps are not made like seat belts.