At around 3 years they can retain enough to be able to boot a system and play with stuff.
I gave the first grandkid an IBM PC and some games when he hit 3. He learned to spell his name with the user name and went on from there. It helped that his dad was heavy into systems so I didn't have to do online support.
And that leveraged into the 6 other kids in the family all being heavy users.
Would you consider getting a refurbed system for the kid's use? They sell, with all the stuff needed, for $150 and under if you look hard enough.