12 year old water heater is getting near the end.
Some tanks last 20 years, but that's the exception.
To find exact age of your brand water heater:http://waterheatertimer.org/pdf/WaterHeaterSerialNumberDecoder.pdfLeaking TP valve
1) Lift TP lever and gently lower back into position to see if that stops leak.
Occasionally something gets stuck in the valve.
2) Sometimes water heater temperature gets turned up because heater is not producing as much hot water as before, and higher temps cause TP to release pressure because of overheating.
Dial temperature down to 120 degrees.http://waterheatertimer.org/How-to-adjust-water-heater-temperature.html
3) If steps above don't fix problem, replace TP valve >> TP valves have 125 or 150 degrees rating >>> older tank shouldn't be running above 125 because higher temps cause more pressure on tank, so thermostat should be lower than 125 degrees TP rating.
Buy replacement TP at any hardware store + a roll of teflon tape to seal pipe threads.
Turn gas control to pilot ... this will keep pilot lit but burner will not fire.
Let water cool, and then drain some water out of tank.
If TP valve is on side of tank, then open drain on side of tank and drain water until it is lower than valve level. Open bathtub spigot to let air into line.http://waterheatertimer.org/Water-heater-will-not-drain.html
Be careful around water that is higher than 120 degrees F >> you can get burned quickly >> apply cold water to any burn.
Use pipe wrench and unscrew TP valve counterclock-wise.
Install new TP and put teflon tape on threads.
Re-fill tank completely and open tub valve on hot side until water is running >> then turn gas control back to original position.
Do not fire up any water heater before tank is completely full of water or this could damage tank.
To read more preventative information about water heater leaks and age problems:http://waterheatertimer.org/Water-at-bottom-of-water-heater.html