If it's well worn but not completely used up at that point, it goes into the city's reserve fleet, to replace newer equipment that's out of service for repairs or maintenance.
After 5 or so years of reserve duty, it's surplus; maintenance at that point is costly, and there is newer equipment falling into reserve status.
So 20-year-old firetrucks go on the market, at a steep discount, and the purchasers typically are small, volunteer fire departments, which could never afford the $500,000 or more needed to buy a new pumper vehicle.
The duty level expected of the old truck isn't nearly as heavy with a volunteer force, where it might be called out for fires only a few times a year instead of several times a day.
The fire engine pictured here was bought new by the Tuscaloosa City Fire Department in 1984; twenty years later, it was sold to the Sapps–Union Chapel Volunteer Fire Department in nearby Pickens County, which is still using it in 2017 and plans to keep running it forever.
Posing with it are Assistant Chief Troy Jordan and Fire Chief Pauline Hall.