Charles Soule is a popular guy these days, because star wars.com has another interview with the “Obi-Wan & Anakin” series writer:
“It probably wasn’t the very beginning, although I’m sure there are some stories to be told there. And I didn’t think it was at the end right before Attack of the Clones, because the relationship that they had was pretty much established by that point. So, I figured it would be kind of in the middle, when Anakin was just on the cusp of becoming a teenager, which is when everybody starts asking questions about the path their life is on. Up to that point, your parents, your guardians, tell you what you’re going to be doing with your life, and you just do it and don’t really think about it that much. But then, right around 13 or so, you start wondering if there might be another way. And likewise, Obi-Wan jumped into this after promising to train Anakin on Qui-Gon’s deathbed, and I don’t think he necessarily knew what training Anakin Skywalker was going to be like. If you had both those questions happening at the same time, you could get a really cool, dramatic story out of it.