Leaking heater or a PRV valve dripping. If it's the PRV is could be for a good reason, how is your pressure?

Prv valave is not leaking pressure same one plumber told me it was a bad heater needed to to be replaced its about 10 years old

Water heaters leak when the glasslike coating wears through inside tha tank. The tank starts to rust and it's a matter of time. The main difference between a 10 year heater and a 15 year heater is the thickness of the coating.