The guidebook goes on to say
The boys served an apprenticeship and on reaching their eighteenth birthday they could either leave to take up a trade or join the Royal Air Force
. I signed on for twelve years "man's service" when I was just over 16 and arrived at RAF Driffield when I was three months short of my 18th birthday; I certainly don't recollect being given any option of leaving when I was 18.
And the dodgy information in the National Arboretum's handbook is being quoted by the guides who conduct parties around the various military memorials.