I believe any form of stretching / warm up is a good thing at any age.
For the really young players / kids it can be quite lighthearted and fun, but is a good way of getting them all together at the start of the session and paying attention.
It also gets them into good habits as they get older, whereby they are not then inclined to want to grab all the footballs and blast them at the keeper, thus delaying the session etc.
I would normally introduce proper stretching (both dynamic and static) and a more intensive warm up from about U12, ending with a game realted activity.
[ November 17, 2005, 06:19: Message edited by: CoachKevinfromPlymouth ]