AA - post #154:
You could join the Army before age 18 of course in those days, but the youngest age he'd have got onto Selection would have been early 20s, as he'd have to have done three years absolute minimum adult service, more like five in his first/parent regiment - I'm not sure many regulars did Selection before the age of 22, so it'd have been 1970 before he'd be in possession of his beige headgear.
Just a small point of order: You could actually have legitimately got your beige headgear (with badge) at the age of 18 (or perhaps younger?) - by way of 23rd SAS which, being a TA regiment, didn't insist of three years prior service. OK - not Regular, but still legit headgear.
Not that, in the 60's, 23 had any remote prospect of being allowed anywhere near any embassy, friendly or otherwise. More likely to be found on a cold, wet German hill/mountain, counting and identifying Russians passing by (and occasionally thieving Warsaw Pact supplies to keep body and soul together) ...