Not aging, but corrosion from urine
Urine so corrosive that it makes salt seem non-corrosive by comparison. Usually the welds at seams and fittings in aluminum tanks are the first to go...average time is 2-5 after the tank first goes into service (which can have nothing to do with the age of the tank if the tank wasn't used for the first X years it was installed on the boat)...pinhole leaks develop, which the boat owner usually finds when he goes looking for the reason his boat is starting to stink. When that happens, it's usually not worth the effort & expense to repair it beyond slapping some Marine Tex on it in the hope of finishing out the season....'cuz the first leak is ONLY the first leak...and sooner or later urine will eat through the sheet metal too...eventually turning the tank into a collander. Stainless is a bad choice for sewage holding too...the welds are just as vulnerable to urine...it just takes a little longer for the sheet metal to corrode all the way through stainless than it does to eat through aluminum. And btw...over-pressurizing due to a blockage won't accelerate the process...but it might cause a leak to make itself known a bit sooner.
