I don't think you can say there's a time limit for tails in the same sense there is for ears-ears generally are done by a certain age to give them the best chance of standing.
Certainly the older a puppy/dog is, the more invasive tail docking is and the more discomfort is involved. Puppies are generally docked at 2-3 days, and at that time the tail is comprised of cartilege that hasn't hardened into bone yet. While I would never say there was no pain involved-the puppies I've seen docked at that age were more stressed out by being restrained than they were by the moment of docking or after.
I've docked rescue puppies at the same time they were cropped and it didn't slow them down for a minute-I'd never hesitate to do it again. It's not unusual for show puppies to be re-docked at this age in an effort to get the length right, or to try to mask a gay tail as much as possible.
I've known more than one older puppy (say 8-9 months) who was re-docked due to the tail being too long. Obviously this was far more extensive surgery-but they were given pain meds to deal with the discomfort it might cause. I probably wouldn't re-dock at this age or older for cosmetic reasons, but I wouldn't do any finger pointing at anyone who did.
