I don’t think there is a correct age to teach someone. I teach children as young as 2, and adults in their 60s and 70s. However the techneque you might use does vary depending on how old the child is.
Children under around 10-11 (and boys particualy) have an amazing ability to engage with a subject if they become passionate about it (think how much kids seem to know about Dinosaurs!). However you cannot force a child to engage at that level, it has to be something they find interesting, and the younger they are, the more important this is.
Therefore when you take young children into the water, the activity needs to be fun. Standing up or turning doesn’t matter, as long as they’re enjoying the experance, they’ll want to do it again, so let their mood guide what they’re working on. At this stage any school with a good kids program is fine, but the best way to make it fun is to do it with their friends.
Older Children (11-12+) are more able to take in information from a proper, structured lesson, and appreciate being treated like an adult. Never-the-less, if their buddies are there too, that will be the biggest draw for them, so try and get some other folks involved or find a school or club nearby that has a kids club attached
If they enjoy the surfing (and only if), provide some extra curricular material; Surf movies, Magazines, and books that they can absorb. Watch a contest (live or online) together and explain how the judging works. I remember reading every page of an old How-To guide I was given over and over when I was pretty young, I lived and breathed surfing, even though I could only surf on summer holidays at the beach.