The T/P would be dry around the threads, what about the bottom of the pipe coming out of it. Put a shallow pan or bowl under it and see if water collect. If it does one of three things possible. You could have a bad T/P or you could have a pressure regulator without an expansion tank or a bad tank. If the water is leaking from the tank, it's time for a new tank in the very, very near future. When tanks go, it is a gradual process, so you're not likely to have the whole bottom fall out and flood the place. Given the age of the tank and the description of the leak, I'm afraid I'd have to bet on the tank being toast, but it won't hurt to check the other possibilities before rushing out to buy a new tank.