Anyone born before 1928 is old enough to have served in WW2, so aged 81 plus as mike said. This is realistically only a generation ago, in that I am in my 40's and my father served in WW2, and my great-uncles who were in WW1 only died when I was in my early teens.
In the 2001 census there were 1,877,000 men 70-79 and 793,000 aged 80+ so coming up to the 3 million mark who were the right age to have been called up - although many of those might have stayed in Britain as Policemen, teachers etc