I got K her first pair of non-flexible soled shoes at 14 months, I think? She started walking at 12 months. Like amc said, it's just a matter of when they need more support.
B was starting to pronate a lot and look pigeon toed when he would run. As soon as we put some non-flexible shoes on him it fixed the problem. Now that I think about it, it was earlier than 18 months- more like 15-16 months. He still has his flexible ones but he's just about grown out of them.