(04 Oct 2020, 1:24 pm)L469 YVK wrote And not forgetting that 6099-6117 have been extensively refurbished and were only 5-5.5 year old when completed. 

At least Go North East do their refurbs properly unlike Arriva who just take an ex London Gemini and tart it up as a premium 'express' bus. Although Go North East are refurbishing 6001-6007 and 6049-6055 which are 8-9 year old, they are a 'heavyweight' vehicle type and subsequently have a long shelf life left in them. Probably wouldn't surprise me if they were kept in front line service until about 12-15 year old.