Actually, that makes sense. If Ian is 40 in 1964 (as William Russell was), then he would have been of age to serve in the latter half of WWII. Okay, I can buy that.
Or earlier in the war. WWI and WWII had a LOT of 14-17 year olds lying about their age and joining up, at least here in the US, not sure if the UK saw the same phenomenon.
Certainly in WWI we did; our three remaining veterans for that war were all either underaged when they joined, or joined right towards the tend of the war. I'm not as knowledgable on WWII and how we mobilised for that, but it's certainly possible.
Like I said, I'm not quite as knowledgeable about the UK's enlistment stats, but the US had that type of thing as late as Korea.