I think Yoda's decision not to train the twins from birth was because he realized that his prior dogma was incorrect. That Anakin was trained at age nine onward wasn't the issue, so much as he was expected to follow all the other dogma as well, such as eliminating attachments that he had already made as a child and simply wasn't capable of eliminating at his age.
I'd also like to think that Anakin's aptitude with the Force is what led to his arrogance, and that also led his fall, among other things. He's basically on the verge of becoming a teenager, and is told that he's the messiah and taught how to use superpowers that are above and beyond what any other Jedi Knight has. By the time of Episode III, he thinks he's more powerful than anyone on the Jedi Council. Delaying the training of his children meant they got through the self-absorbed age without having that sort of power.
When Lucas wrote Episode I, the reason that he wrote Anakin so young was that he wanted the separation from his mother to be a traumatic event in his life that helped define him and his flaws that led to his becoming Vader.
Lucas seems like a decent writer when it comes to writing the story, but when it comes to the nitty gritty of dialog is where he fails. I think he actually had a really good setup for Anakin to become Vader with his attachment to his mother and the absence of the father figure. Qui-Gon comes to take on the father role for Anakin in the movie and behaves in a rather fatherly way. Through the course of the movie, he loses both figures as Qui-Gon dies and he's separated from his mother. So no wonder he becomes clingy, and fears about his mother's well-being.
To some extent, Padme also becomes the mother figure in Episode I despite being only slightly older than Anakin, and I think that's where he begins transferring that sort of attachment to her. The void left by Qui-Gon is filled by both Obi-Wan and Palpatine. I think that from 1994 onward Anakin's characterization was basically set in the sense that Lucas had decided upon that sort of a story, and it follows from Episode I that Anakin would become an overconfident teenager with attachment issues.
Edit: Yoda's protest that Luke is too old seemed like it was another test of Luke, even before the prequels, IMO. Leia and Luke are their last hopes, they admit as much in the same film. There's no way he's not training Luke.
Edit 2: I forgot to add with Obi-Wan and Palpatine becoming father figures, that Obi-Wan becomes the kind of father figure that teens have that they hate because he undoubtedly looked up to Obi-Wan from when they met onward in Episode I, and by the time he became a teenager and older he started to feel that he was Obi-Wan's equal and realized that Obi-Wan wasn't infallible as he had thought as a child, as most teens do with their parents. Meanwhile, Qui-Gon exists as sort of an impossible father figure to live up to because he only has his brief positive experience with Qui-Gon to compare Obi-Wan to, which Obi-Wan can't possibly meet. Palpatine escaped this sort of criticism since all he did was feed Anakin's ego, and probably didn't have as much time spent with Anakin. And his mother and Padme never saw Anakin during this phase of his life.
"Trust not the words of a poet, as he is born to seduce. Yet for poetry to seize the heart, it must ring with the chimes of truth.”
“The world is understood through metaphors. Language is a metaphor-system. Mathematics is a metaphor-system. All real-world schools of magic and religion revolve around the understanding of vast metaphor-systems, symbols as they relate to concepts."
"See, the thing is, everything everyone tells you is a lie. The truth is always bigger than the words we use to describe it."
Last edited by Crash Override on Wed Jun 22, 2011 8:31 pm; edited 1 time in total