I was really annoyed when Nottingham closed, they made so much stuff there, it was so close to me, I wanted to see TV shows made, but you had to be 18 to get into the audience. And a matter of months before I hit 18... they closed!
Hate to say it, but I think you might have been misinformed by someone...
Looking at the terms on the back of the tickets I've got handy, all the shows had their own age limits - Family Fortunes was as low as 10 years old, Barbara was 14, and Kiss Me Kate was 16.
(though technically I shouldn't have been seeing KMK as I was only 14 at the time... but then I looked much older and I don't recall them ever checking anyone - and being repeat visitors we got quite friendly with the audience admin).
Can't imagine anything there having an age limit of 18, other than Central Weekend I guess.