Nutty,
15 years service for the medal The thing that always got me was that time served under 18 never counted.
The good news was that Boy Entrant/Apprentice conduct sheets were deleted on passing out!
It was just too easy to get put on jankers, almost a ritual of passage.
The really odd time was the 6 months between 17½ and 18th birthday. It counted for some thing but not others.
If you stayed in till pension, it counted as pensionable service. I didn't and it is in my discharge book as 'previous service'.