To me it's not so much about the age or athleticism of any pickups.
What we need to avoid are guys who are themselves injury-prone or out of shape. Looking back, this has been the curse of Danny's prize pickups (Sheed, JO, Shaq, etc).
With an old core we need supporting guys who will play 82 games and can start 10-15 games when the inevitable injuries hit during the season. Wasting another MLE on someone who plays 20 games is what we need to avoid.
So, Jason Terry, though 34 , is very durable and does not concern me as much as Rashard Lewis, who is very fragile at 32.