Same experience: my 4yo built a rainbow-colored case for a Pi2 B, cabled in monitor and keyboard and mouse and power, and knows it’s the computer she built. Following Goerzen’s advice, she gets a text login—so three years later, she knows about startx, apt, and finds alpine about as usable as iPad Mail.app. When the computer was disassembled for a move, we had a great conversation about how to put it back together, what was working where, and how USB ports defy physics and have three sides, one of which is invisible.
He did it this past summer, so he had just turned 8. Those kits aren't hard at all if your kid is already used to stuff like Lego. My son builds stuff intended for 14 year olds no sweat.