My boat
has an 11.5 gal tank that I keep full most of the time. I use the engine for about 50-60 hrs per season and typically will use about 12 gal of diesel. I tend to refill the tank when it down 2 gal or so. I do this thinking that I keep "refreshing" the older diesel in there with new diesel. I also use "Pri-D" as a fuel stabilizer/conditioner and add a dash of anti-algae stuff too. As such, not sure what the average age of my fuel is but I have had no fuel-related problems with my Yanmar 2GM20F at all. FWIW, a friend of mine with a Catalina 27 and a Universal diesel with a 20 gal tank tells me that he has not put any fuel at all in his boat since he filled the tank 3 years ago. He, too, claims no fuel related engine problesm.... Go figure.