There are a few reasons why your water heater safety relief valve may overflow. The water may be getting hotter than the valve permits, so it opens to prevent an explosion. The relief valve may be getting weaker and can not hold the pressure any longer. The pressure in the tank may be getting too high because an expansion tank has failed. Something new, such as a check valve or backflow preventer has been added to the system.
The safest way to find out what is werong is to have an experienced plumber look over the water heater to find the cause. He can be asked to install a new relief valve during the call to him just to make sure the valve is not bad. Then he can suggest whether a new water heater is needed. If the water heater is that old, it would save you some money to have it replaced now instead of waiting for the most likely occasion of a leaking heater.