My son has navigated my Macs well ever since he was about 5, which was when he was coordinated to use the mouse. I try to expose my son to multimedia apps which aren't too menu-heavy. Stuff like Spongefork, where you can draw squiggles which are rendered as audio. And Pixeltoy, where you can draw shapes and colors which cycle through a generative video display. Simple drawing and painting apps. Anything which gets him to interact and be creative rather than passively consuming books and videos.