To take your comments at face value, you are saying that buses bought for 2 x 7 year contracts could undertake a third 7 year contract and be the best part of 20 years' old. I have seen a mix of buses on various contracts - for example, the Roundabout routes in Orpington have a mix of 12 and 64 plate vehicles, for contracts started at the same time.
There will be a time when the buses are time-expired for London - too expensive to maintain, refurbish etc, and new buses will be required. To take the Roundabout example, when the contract is up for tender in 2 years' time, the current buses will be too old to service another 5 or 7 year contract (not due to any age restriction, but because they will be knackered.) If Go-Ahead do not have any other suitable existing vehicles in their stable, either they will not be able to bid on the basis of 'existing vehicles', or they may hope to pick up some second hand examples from other London operators, but the latter is far from certain. They can hardly buy these buses from Ensign when they submit their tenders; the have to wait to see if they have won the tender, then try to source these vehicles. My point is that there may not be any such vehicles available, and therein lies the problem.
Of course, the tender may require new vehicles, which will mean more expensive electric vehicles. Given the state of TfL's finances, this is unlikely to be an option for all tenders.