I think it might be ever? or in prison, i don't know but that's what that cited report claims. (if prison, stats don't match up)
I too was surprised, but 30%... I mean most adults these days were teens between late 60s and late 80s. Prime times for LSD & MDMA use I do believe.
50% of young people? Maybe if alcohol before age of consent counted, would have thought it would be higher than that especially in uk, but without it a lot lower - maybe 3%.
Anyway, good luck in your exam. Hopefully a mod can replicate/move this thread to EADD where the UK people reside. My internets so poor, i've tried posting this like 15 times and still no result, I apologise in advance if i multi-post.