The bus reimbursement program in WA state is rather convoluted but the short story is this:
You purchase a new Type 'D' bus. It is put on a 13-year reimbursement program from the state. Each year you get 1/13th of what it would cost to purchase a replacement bus that year. At the end of the 13-year cycle you should have enough $$$ to replace the bus. To keep the districts from spending those $$$ on other stuff the $$$ are held by the county treasurer and the only way in which the district can access those funds is by submitting an invoice for the purchase of a new bus.
At the end of the 13-year cycle it doesn't make any sense to keep a bus. Not only is it not generating any $$$ from the state but it is now getting old enough to start to nickel and dime the district in thousand dollar bills.
So in answer to your question, in WA state you are not required to retire a bus by any specific age. It wasn't that long ago that East Valley in Yakima still had some 1950's and 1960's vintage Gillig buses that were still passing the annual state patrol inspection.
There are still a handful of buses in the state that were purchased on 15-year and 20-year schedules. But as of about five years ago the longest schedule is now 13-years.