I have never had a colonic. I imagine it is similar to an enema.
As for the youngest age, I would suspect that it depends on how long you are cleansing for. A year or two after puberty when the body is about 99% done with the growth process I can't imagine a 7-10 day fast would hurt at all. But as a teen, since their metabolism is so high, it is probably best to eat right and exercise and start yourself on the correct road to healthy eating.
Prior to that, I don't see any reason to deprive a growing child of food. If the child has a weight problem, this is the time to teach him or her how to eat and exercise properly, not starve them.