Thankfully, the DFT have some stats on this one. The stats available are for financial years up until financial year ending March 2023. (
Link to stats. Note that you have to download dataset BUS06 and it downloads into a spreadsheet). You'll need sheet/table BUS06f, titled 'Proportion of buses by year of manufacture used as Public Service Vehicles by metropolitan area status and country [note 9], local bus operators only [note 1]: Great Britain, year ending March 2023'
Buses manufactured between 2018/19 financial year and 2022/23 financial year (the oldest buses here would be 6 years old)
London has 15.5%, Scotland 29.9%, Wales 10.5%, England excluding London 11.8%.
Your other point for electric vehicles, that can be found in the same spreadsheet but switch to sheet BUS06e titled 'Percentage of buses used as Public Service Vehicles by emissions standards and fuel type by metropolitan area status and country [note 9], local bus operators only [note 1]: Great Britain, year ending March 2023, figures as at 31 March'
10.6% of Londons buses are electric, 11.4% of Scotlands buses are electric, 5.4% of Wales, 1.5% of England outside of London.
In both cases, the numbers will have changed as there have been a lot of buses delivered over the past 16 months. In November we will get updated stats for the 2023/24 financial year.
With regards to your general observations, I think that sometimes it can seem like there are a lot of new buses operating, there will also be a lot of old in the wider areas. Some buses also don't look as old as they actually are, if they are maintained well. For me, that is things like Streetlites still look modern, some of these are 15 years old now but to me, if the livery is smart and modern, I probably wouldn't be able to tell a 2010 Streetlite from a 2024 one. Looks can be deceiving if the bus is looked after.