There is no age requirement for the petting zoo. I don't recall seeing more than one CM standing around monitoring the petting zoo (which is outside of Rafiki's Planet Watch - you take a short train ride to get there). It's a pretty standard little petting zoo, with lots of sheep & goats and I think a donkey. They have brushes that you can use to brush the animals. One thing to keep in mind, is that there is very little shade in this area and the ground is very dusty, so it gets VERY hot quickly. My kids, who are HUGE animal lovers, don't even like to go to the petting zoo any more because it's so hot and frankly, the one at our local crummy zoo is better.
The rest of the Planet Watch area is great though. Try the sound booths where you sit inside and listen through headphones - it's relaxing for the parents and even the smallest kids get a kick out of the surround sound.