This chap is wearing AMPC slip-on titles and his row of medal ribbons suggests something more than simply inter-war service. I believe that they recruited up to 50 years, or older if there were special skills.
Most armies had done so for years, for example in my family history I have a Gt Gt Uncle who went back to the colours in his 50s during WW1 because the RE needed men with light railway experience and they shipped him out to Salonika as a Sergeant (where he caught malaria) and I'm sure similar things happened in WW2 but the numbers would be very small.
Lt Col Tod of the 2nd Bn Royal Scots Fusiliers served and was a POW in WW1. He was captured alongside much of his battalion on the 28th May 1940. Uniquely he was the senior British officer in Colditz, earning the MBE for his efforts to provide and protect his fellow prisoners.
Also in the RAF was Louis Strange who AFAIK was the only man to win a DFC in both world wars - the 2nd when flying a Hurricane during the Battle of France