I think that if properly supervised, there is no reason 10-year olds can't learn how to use 3d printers. There are plenty of 8-year olds who know not to touch hot stoves, etc., and are mature enough to understand safety and cause and effect. Sure, you can get a nasty burn from a printer, but nothing life threatening. And, I don't care how young you are, if you get burned once, it's unlikely you'll make that mistake again.

The important thing is to get kids involved in technology as soon as possible. The more years they have to interact with the technology, the better they can think outside the box by the time they are in their early teens. If we wait till they are teenagers before we expose them to these technologies, then they really don't get rolling and innovative until the end of high school.