Depends on a few things.
Can you afford to, in the Win Loss Column, start the Younger Guy? How much of a drop off in Talent is there? How good is the Vet?
Say the Vet is better on Current, but still just meh... but the Rookie can develop from **** to Great. Then I put him in, get that Positional Experience building and don't look back.
But if I have an All Pro Vet, he's playing. Rookie needs to sit and develop, get Pre-Season Snaps, rotate as depth, things like that.