According to my calculations, Milton Keynes in Buckinghamshire has the newest housing stock of any major town or city in England and Wales. This should not come as any surprise, of course – Milton Keynes was in the third and last wave of new towns formally designated between 1967 and 1970.
Milton Keynes is the only town with a median build period of 1983-1992 for its housing stock.
The next ten towns with the newest housing stock, all in England, have a median build period of 1973-82: Crawley, Basildon, Swindon, Salford, Colchester, Redditch, Peterborough, Telford, Basingstoke, Bracknell.
(Basildon, Peterborough, and Bracknell are also "new towns".)
Conversely the major towns with the oldest housing stock are Southport and Brighton and Hove. Both have a median build period of 1919-1929, but Southport's distribution is slightly older.
The next oldest towns and cities, all with a median build period of 1930-1939, are: Bath, Liverpool, Manchester, Hastings, Stockport, Southend-on-Sea, London, Halifax, Birkenhead, Portsmouth, Blackpool, and Burnley.
Here's the complete list of major towns and cities ranked by housing age.
I calculated these rankings by aggregating LSOA-level property statistics, released by the Valuation Office Agency (VOA) in November 2018, into ONS's Major Towns and Cities geography, via a best-fit lookup table.
The aggregated figures are not precise, as there is some rounding and suppression of small numbers in the LSOA data. However they should be robust enough for the simple purposes of these rankings.
All three source datasets are open data (though VOA needed a little encouragement).
If you want to explore VOA's build period statistics in more detail, there's a free analytic data product called Age Profiles of Housing Stock on my Datadaptive website. There's also a lookup tool.
Attribution statement for the boundaries in the map images above: Contains National Statistics data © Crown copyright and database right 2019 and contains OS data © Crown copyright and database right 2019.