Mine is three and a half and we've been playing through Super Mario World together. Sometimes he wants a go and sometimes he wants to watch me. He'll also watch me run round time trials on Mario Kart and has played Sonic (which is a little simpler in terms of controls than Mario). He has fun but he can also get a little frustrated because the games don't always conform to what he wants (he always wants a rhino on every level of DKC).
I've actually had a lot of fun playing PC games with him. He likes watching Plants Vs Zombies and giggled like an absolute loon playing Untitled Goose Game (or:
The Game With The Goose Who Steals Everyone's Stuff ). I actually logged close to forty hours on Jurassic World earlier this year unlocking all the dinosaurs so I could build him his own dinosaur island we could explore together - he LOVED it!
I think sitting and actively playing (preferably together) is a lot better than just sitting on YouTube. He's currently in this routine where he'll come into our bedroom in the night, then after about 6:15 he can have some tablet time. Getting him off the tablet to go to nursery is a nightmare. The zombie comment is right. What's weird is that he'll watch ten seconds of a video then change, then change, then change. It's like a compulsion! I feel bad that I give him too much screen time on my days off with him but in mine and other current parents' defence we've had no children's groups, no soft play, and largely gooseberry fool weather SO WHAT THE strawberry float DO YOU DO WITH THEM?