My 7 year old loves watching the videos on YouTube. He wants to try to build on his own. It's difficult to tell how easy the game is to maneuver through.  What age do you think kids could start playing this on their own?

And can it be downloaded on a tablet or does it require a full computer with windows?

Thank you!

I'll be as helpful as I can as a childless person ^^
I don't really have much data to go on as far as a recommended age, but here's links to a 7 year old and even a 5 year old enjoying the game.

When I showed the game at a gaming expo children who seemed ~7 were able to pick it up, with perhaps a bit of poking from their parent to read the text on the screen telling you how to play. Once they get it they didn't seem to have any problems.
The only issue I noticed was some younger kids having trouble using both the mouse and keyboard simultaneously, but this isn't even a requirement and they adjusted by first moving the 'player' with the keyboard and then using the mouse to look and put down pieces. Track building is done entirely with the mouse.

At the moment the game only runs on Windows PCs, but I'm looking into expanding the platforms it's available on.

Hope that helps!