In the early 70s, when I was a fanatical, young, teenage caver, the lower age limit for many Mendip caves was 16. GB, Longwood and several others were therefore out-of-bounds for several years. Interestingly, Reservoir Hole was allowable and I first went down aged 11. Granted, it was a rather shorter trip in those days...
By the time I reached 16, I'd largely switched to climbing as there were no real restrictions and you could decide how much adventure you felt you could cope with, rather than having to accept the more limited adventures imposed by access agreements.
I'm not clear on the legal arguments for limiting access to over 18s. However, I suspect that the most productive time for recruiting new caver to the sport is before or around puberty (before other attractions/distractions take over). If there only a small number of really worthwhile caves in your area and they have to last for several years before you are old enough to go down the others, you are likely to start seeking excitement elsewhere. Therefore, while I support the idea that certain caves should have access restricted in the interests of conservation, denying access to competent teenagers (supervised by a leader) is not in the long term interests of the sport.