Another vote for 'never heard of it'
Blackpool may have had the oldest buses in regular service come deregulation - their 1967-68 PD3s were still on the road, and older Routemasters joined the fleet around deregulation time.
(Although there may have been older rear engined buses still on the road but less obvious!)
I think at least some of Burnley, Southampton and Southend bought Routemasters while still council owned following deregulation, and Maidstone's semi preserved PD2 ran in regular service for a while in the late 80s.
Post 1985 Act, the only sort of thing I can think of where an age limit came in was that some tendering authorities (counties / PTEs and so on) did put age limits in their contract specifications as a fairly blunt way of trying to improve quality on tendered services.
But it would have been (and fairly sure it still would be) illegal for a tendering authority to impose different conditions for different operators / different types of operators.