I'd like to thank everyone for some great comments and input.
councilman24 made some excellent points and I just want to clarify one thing you said...
As I understand it, parents and legal guardians can sign waivers for the minors they are responsible for and these are legally admissable (I'm not a lawyer but I am a parent) otherwise schools and other organsations who take kids away canoeing or climbing or on other 'hazardous' activities couldn't operate (I know, don't beat me up on the definition of hazardous, its not mine).
Not meaning to be picky, but you didn't really mean to say that did you? There isn't some dark legal concept somewhere that we don't generally know about is there? having said that, riggerrob in his post said something similar about a parental consent signature 'not holding up in court'. Does anyone have any links to anything which gives details about this? If there are really any precedents, most grateful to receive them.
JerryBaumchen makes a good point about 'shop around and if you work at it, you may find it'. Thats interesting that Oregon dropzones are 33% for 16yrs and above and 66% for 18yrs and above. I wonder how the other states break down and whether Oregon is representative? (I know, too small a group to apply that to all 50 states but its a start...)
CanadianFella highlights the individuality of Quebec province, and its worth noting that the 14 year old jumping perspective also fits with Australia (14 minimum, and right now trialling a waiver system to reduce the age to, well, no lower limit but just based on parental consent and a written application by the child to the Director of Safety). The rest of the Canadian rules in respect of lower age limit seem to be similar to USA.
This is just trying to summarise the situation so far and clarify a couple of points. Once again, sincere thanks to everyone for input and more data will be appreciated.