Ralph - good for you!
I agree with Phil's comment too - vertical caving generally has more risks than horizontal caving, no matter what age the participants are. But I think vertical caving can be an appropriate activity for the group Ralph described, if the risks are properly managed by someone who has the time and skills to do so. Incorporating above-ground vertical training and assessing individual aptitudes is a good way to start, along with doing regular horizontal caving trips. Then when the group is ready, bring all the skills together.
Personally, I never went caving as a Scout while growing up in New Zealand - mostly because I left Scouts by age 14. But later I assisted with Venturer Scouts (similar to US Venture Scouts, ages 14 to 18 ), and their activities do include caving. I helped lead a number of vertical trips in caves and disused mines (stable mines - not like the ones in Nevada), and the kids had a great time. In fact, there is a National Scout Caving School that has been going in NZ since 1978 -
http://www.scouts.org.nz/schools/caving.htm They teach vertical caving, and I know a number of dedicated cavers who got their introduction to caving there.
I've also helped out with Scouts and other youth organizations on caving trips in the US. No vertical caving per se, but there was some above ground climbing/rappelling incorporated in one program. Most kids had fun, some couldn't wait to get back home to their Playstations, but one or two might grow up to be actual cavers. On one trip, a child's father had more fun than his kid, and decided to come on some grotto trips!
It's a pity there isn't a similar caving school for Scouts in the US - but I guess that would just give lawyers a feeding frenzy. It's a shame when a little common sense and basic risk management can't be trusted any more, and instead the poor little children have to be sheltered from having any adventures in the real world...