It really depends on how much of an affinity he has for video games. I bought my son a Switch when he turned 4 along with Mario Odyssey. He had been playing video games since before he turned 3 and started with Yoshi's Woolly World on Wii U, before later moving into Mario 3D World. Mario Odyssey was hard at first for him (primarily controlling the camera with the right stick), but he figured it out pretty quickly, and, over the course of a year, got really good at the game to the point that he is now probably better than me.
Around the time he turned 5, he started showing an interest in Zelda and I let him play on my save file a little bit to see if he could figure it out. Turns out he could! It wasn't simple at first, but he totally fell in love with the game. I ended up buying the digital version and giving him my cartridge so he could start a brand new game on his own Switch. I had to help him out a lot in the beginning, but now, almost a year later, he's completely mastered the game. I helped him through his play-through by looking up quest solutions and such online and telling him what to do next sometimes, but in terms of controlling the game and engaging with the mechanics, he's no worse than I am.
We've since gone on to do absolutely everything in the game and its DLC other than collect all the Korok Seeds and he is a total badass when it comes to the combat. Even though he's done everything, he still just loves to run through the world and mess around with the various systems. It's easily his favorite game alongside Minecraft and Pokémon right now.
