I wonder what age of bus you deem to be old, TSR?!
Personally, I'm quite impressed with National Express' West Midlands seeming continual investment in fleets of new vehicles: Recently, a further batch of new Scania Omnicity single decks have replaced the Mercedes O405s that I used to see up and down the Soho Road, which were of 1996/7 vintage, and most recently a number of new Enviro 400s have entered service on the Walsall Road corridor, joining those that have operated the express services on that route for a few years now, which have replaced Geminis which were only introduced in 2002/2003 IIRC. This appears to leave the Wright Crusader bodied Volvo single decks as the oldest vehicles in the West Midlands fleet, which are 12-15 years old and withdrawals of the oldest ones appear to have started.
So based on this experience, National Express West Midlands undertake fleet withdrawals when vehicles reach around fifteen years old, which seems to be in line with the majority of other operators in provincial cities around the UK. There's even new double deck Enviro hybrids on the 22/23 route now as well, keeping pace with developments in other cities including Sheffield, Edinburgh and Newcastle.
I also never fail to be impressed that NXWM operate such a large fleet of modern double deckers that is being continually added to: That's a breath of fresh air when you come from Sunderland, where all the Stagecoach operated (former municipal) services that used to be run with double deckers have been downgraded to single decks, albeit with the latest form of Dennis Darts and the like, in recent years, as have many of the Go North East operated routes! At least in that area though Newcastle are moving in the right direction with a fleet of new Enviro 400s, and even hybrids, replacing Dennis Darts.
I do have to say that the interior condition of the majority of NXWMs' bus fleet often leaves a lot to be desired, and it does give the impression that they are not as well maintained as many other fleets around the country: When I was in Edinburgh recently, I was astonished at how well presented all of the Lothian Buses vehicles that I travelled on were, and even back in Sunderland both the former municipal operation now run by Stagecoach and the former NBC interests now run by Go North East demonstrate much cleaner vehicles than those operating in Birmingham. It is however possible that this has more to do with the class of passenger using the buses in such a large city as Birmingham that contains areas suffering from high social deprivation than with maintenance regimes.
It has often seemed to me that the vehicles operated on the routes of the former municipal operator, which is NXWM for the West Midlands area, often appear more uncared for than those run by the companies that have taken over the former National Bus Company operations. This is probably down to the nature of the routes, and the breadth of area coverage, that the municipal operations had to cover that were never going to be as lucrative as those operated by NBC organisations.
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Also, it probably doesn't help that the vast swathes of Volvo B7s that NXWM operated are, IMO, utterly crap, gutless things that already seem to be well and truly past their best at barely ten years of age. The sooner that they are all replaced by Enviro 400s the better if you ask me, I'm absolutely fed up of the things wheezing past me!