The BBC article on the last man demobbed from National Service says he was 22 in 1960, and that some of his cohort were 4 years younger than him.
So that last batch included men born between 1938 and 1942
I must have added up wrong in previous post (by 18months, as I think it's duration I mucked up - peacetime conscription was initially 18months, but was extended in 1950 because of the Korean War).
Despite what first hit on google says, the youngest on the final cohort would be 80 turning 81 this year.