I started them young, they were 3 and 5 the first time they put together a Raspberry Pi. They were Pi 3B+ and they had clear acrylic cases with tiny touch screens. They both slept with them bedside for a few months. I wrote a program that changed the color of the clock on the display. Between 7pm and 6am, the clock was dim (red I think), which meant sleepy time. It surved as a nightlight. At 6am, it changed to blue, which was the signal that it was ok to get out of bed. They ended up in a box, and eventually recycled into other projects.
After Christmas 2018, while still on winter break from school, we did our second builds. This time, they both had a friend joining the fun. My kids were 5 and 7, and their friends were both in the same class at school. We all went to Microcenter and bought Raspberry Pi Zero boards. Each kid picked out their own case, to make it unique to them. This time they had to install Raspian on the SDCard and test it themselves. They used my laptop to burn the SDCard. Then they all shared the same keyboard, mouse and monitor to setup and test each build. At the end, I quizzed them on the different ports, and had them explain the different parts of the computer. For about $20 for each kid, that was a solid 5+ hours of learning, tinkering, building computers (and driving).
February 2020, my oldest are now 6 and 8, we had a day with nothing on the calendar. We went back to Microcenter, which we now call “the robot store”, and picked up some more stuff. This time its Raspberry Pi 4, with 2GB of RAM. We bought LCD screens, the official Raspberry Pi keyboard and mouse, and a case which housed the screen and board. This time my kids are old enough for Minecraft. After they put everything together, their “test” was running Minecraft “Pi edition” and playing for a bit. They have since figured out that they can also go to YouTube on these now. I’ll circle back with my answer to kids on YouTube later. This time, they have also started actual programming. Out of the box, Raspbian comes with a Python editor. Both kids have walked through the basics of programming Minecraft Pi using Python. The highlight for them, was programming huge cubes of TNT and making it explode. The highlight for me, was them programming.