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One of Yoda's objections to training Luke was that he was too old. Also, with the limited time available, by the time Luke first faced Darth Vader he was not ready for it.

Would it not have been a much better plan on the part of Obi-Wan and Yoda to train Luke from a young age and send him in to join the rebellion as a fully trained Jedi knight?

marked as duplicate by F1Krazy, NKCampbell, Valorum star-wars May 18 '18 at 20:50

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    Yup, that post's answer covers it. – user100501 May 18 '18 at 20:27