The only way I can imagine yoga being unhealthy or unsafe for anyone of any age is if the teacher is unqualified. I have practiced yoga for over 15 years, taught at times, and have a private practice at home that I share with my five-year-old. He has been more interested at some times than others, which is fine with me. Yoga is good for the body, mind and spirit. It is a wonderful discipline to introduce to children. I recommend "The Children's Book of Yoga" by Thia Luby if you want to explore ways to make yoga fun and age appropriate for children.