I'll go ahead and answer your excellent question while you're working on that poll.
Realistically, it would depend on the kid. I know some 30 year olds I wouldn't trust near anything vaguely fragile, and I have known some five year olds I wouldn't be particularly concerned about.
If I have to pick a general rule of thumb ... I'd probably say 6th grade, about 11 years old. At that point here in the States they're starting middle school and having to carry books from class to class for the first time, so it's a logical entry point. Also, I'd think a majority of 11 year olds have grasped the concept that some things are expensive and have to be treated gently.
If/as the durability of the devices goes up, then the age might drop, but then again, it might not, because I'm not sure how useful it would be before that educational stage.
