My personal guideline is a minimum around 12-13 years old. At this age they're old enough to have a comprehension about the potential dangers and be able to concentrate on things for more than 10 seconds at a time (so they're more likely to follow the rules you lay out about holding on, not goofing around, communicating with you, etc), and (most importantly) they're less likely to get bored and just fall asleep.
