How old is too old?

I have a boat with an 80 gallon tank. I've never ever had it full and I've owned it 10 years. ONCE, I didnt start it for 9 months. Well.... That led to 2 new Fuel Pumps and a $900 Bill !!! (The Fuel Varnished the Pumps).

Now, I only put in what I can use within 6 months and I use Either Sta-Bil or Pri-G Which Helps the fuel last a "up to" a year.

Personally, I just Blend my old gas with new as well around 10 gallons fresh to 1 gallon of old. If I use it in a car. Maybe 5-1 in my lawnmower, It will still burn.