I'm in the camp that says Yoda learned from the past mistakes of the Jedi, that being mindful of the future and not listening to the "here and now" of the living Force.
While it's not said in the movies, or even in Lucas interviews, I think that while there's a danger in training someone when they're old enough to have attachments, there's also a great potential for greatness.
Someone like Luke who lived to adulthood would have a wider range of "real world" experiences that once integrated with full Jedi training, would have made him far, far more powerful than your standard Jedi. For example, it never occured to any of the generations of Jedi trained to not question the Jedi relationship with the Senate and Chancellor to question that as the possible source of discontent. For someone with experience of growing up on a nearly lawless planet, and experiences of close betrayal might naturally ask that kind of question.
The problem, of course is that it's much harder to train these Jedi. And there's a greater risk that they'll turn. But in ESB, Yoda says something that we don't hear in the prequel area and given the circumstances, wouldn't expect to hear, "You must
unlearn..." Knowing that their students have some baggage that they must fix is the key to successfully training those not raised completely within the confines of the Jedi Temple and one that was completely lost on training Anakin.
While it's never been spelled out completely like this, I believe the Jedi history has had several epochs in their history that pointed to this downfall:
- First. Small groups gather to master the Force. At exploritory stage, largely neutral. This was mentioned somewhere that they began as philosophers of the Force.
- Second. A larger, centralized group that collectively decides its in the best intrest of beings in the galaxy for them to act as agents of good and does so on their own.
- Third. Realizes they could do more good under the sanction of the biggest political force and agrees to work under the Senate as a police force.
- Fourth. Due to the demands of the galaxy, needs to ramp up their size and that's where they realize it would be easier to do this by training Jedi from childhood instead of the various stages they'd been used to up until then.
- Fifth. By only raising as children, the Jedi lose their knowlege and experience (which is largely based in listening to the "living force") in how to train Jedi a) of all ages and b) dealing with those who explore the dark side. As a result, the rise of dark side Force uses is allowed to exist. This epoch would be from the Tales of the Jedi era until the extermination of the Jedi in ROTS.
One of the few things I like about the post ROTJ novels, even after more origins of the Jedi were revealed after TPM, is that Luke trains a wide range of folk, bringing the Jedi back to what I'd consider their 3rd Epoch.
