I am occasionally taxed because Peter Cotton is made an acting Lieutenant-Colonel in his twenties just after the end of World War 2.  This doesn’t happen now. No problem there.
What is a little curious is that some 21st Century readers believe that because they are incredulous, such a promotion at such a young age couldn’t have happened in 1945. It’s an extraordinarily confident opinion, sometimes aggressively expressed - but I’m afraid it is not backed by any research whatsoever.
 In World War 2 promotions could be astonishingly rapid. To name someone people may have heard of, the late professor and contentious politician Enoch Powell went from private soldier to Brigadier (a much higher rank than Lieutenant-Colonel) without ever seeing action. He became a Lieutenant-Colonel in 1942. Powell was nearly thirty then – but my point here is the rapidity of his promotion. Private, Corporal, Sergeant, 2nd Lieutenant, Lieutenant, Major, Lieutenant-Colonel, Colonel, Brigadier in six years. The only other man in the British army to do similar was Fitzroy Maclean.
But a relatively large number of people did become Lieutenant-Colonels in their twenties. If they continued in the army after the war, their rank was often reduced to Major.  War speeds things up. Peace slows them down.
In Peter Cotton’s case, he is embarrassed by his promotion but it is an intelligence/ diplomatic one, to give him a certain rank in Washington DC and enable him to do his job. People’s use of his title is a courtesy, sometimes with a little irony - and he does not use it himself.
The best I can do is to apologize for upsetting some readers’ views of what they think should be believable. But when forty is regarded by some nowadays as young, those doubters should just step back and think. Perhaps they should consider the other armed forces. Guy Gibson of Dambusters’ fame was a Wing-Commander (the equivalent of Lt-Col) at 23. Similarly there were Naval Commanders of that age.
So what can I say? Young men were asked to grow up very quickly in WW2 and to take on responsibilities and duties not associated with peacetime. Cotton’s father considers that his son’s generation has ‘missed out’ on growing up, have been put through a crash course but not prepared for life outside war.  I think he has a point when he calls them a sort of ‘lost generation’ – sometimes with, in times of peace - absurdly high ranks.