certainly this was the case when I was a Cadet - of all those I started with after 2 years there was less than half still attending, and by the time i aged out I was one of two remaining.
I think it is “natural wastage” while someone will give something a go, after a year if it isn’t for them they move on.
and with teenagers there are so many other draws to their attention - Less so at my current unit, but certainly see drop offs at 16 when Cadets get into the world of work, be that full time or more likely part time /weekend work.
then of course there is the academic pressures. at 15-16 many choose to concentrate on school, and so see a drop in attendance there - then again at 17 when collage/6th form comes around.
if an individual isn’t questioning what they get from the organisation the chances are they are not an NCO, not attending camps or weekend courses, or simply the events run by Sqn (be these community based fundraisers, or PTS training or otherwise “syllabus stuff”) then it is no wonder they leave