There's really no need to replace the decking unless it's damaged, regardless of it's age. It would be an unnecessary expense, and a large one a that.

That vent pipe is PVC which tells me it's a lot newer than this house so my guess would be that whenever that vent was added or replaced they destroyed that board in the process. Regardless, the roofers should have corrected it. It's possible they just forgot to circle back to it if this was the only issue.