If it helps, the bus age profile is in the annual TfL bus fleet audit.
Normally TfL bases its fleet on 14 year London life, so if add 14 years to introduction date, get an approx withdrawal date. However the age profile is not very even, there are lean and busy renewal years.
Quoting the most recent (at 31 March 2025) fleet audit :
2015 buses 164 diesel, 609 hybrid, 0 electric (total 773)
2016 buses 247 diesel, 722 hybrid, 49 electric (total 1018)
2017 buses 305 diesel, 676 hybrid, 18 electric (total 999)
2018 buses 199 diesel, 478 hybrid, 66 electric (total 743)
2019 buses 174 diesel, 203 hybrid, 98 electric (total 475)
2020 buses 13 diesel, 130 hybrid, 170 electric, 4 hydrogen (total 317)
2021 buses 0 diesel, 0 hybrid, 318 electric, 26 hydrogen (total 334)
2022 (and later) all buses electric
Clearly unless TfL intends to retire lots of diesel and hybrids early and replace them by expensive electric buses, they are going to have diminishing number of diesel and hybrids until about 2034
So TfL have to make a decision at some stage if they want to renew contracts for diesel buses in 2027-28 (even if only for extra 2-5 years)