Well I got my first PC when I was 5 (2005), and I just learnt to use the PC by myself (because my parents didn't understand it) over several years.
When I will eventually have a child, these would be my steps to encourage them to use a PC:
- Tell them that they can play paid games for free and show them how to do it
- Give general internet usage tips
- Convince them that a PC worths way more than a console, because you can play way more games on it, and you can also do a bunch more things with it
I don't think a 5 year old will understand a PC's usefullnes for sure, but their receptors will definitely be turned on after hearing FREE GAMES.