If your towels have gone stiff and stopped soaking anything up, it's usually fabric softener rather than age. We don't use it, and we don't over-dry, so the pile stays soft and thirsty.
Bath, kitchen, and gym towels all get the same treatment. If yours have already gone stiff, a couple of proper washes usually brings most of it back. Drop them off at $2.00 a pound, or run them yourself in machines big enough to give them room.
