system
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human
[Star Wars] If Yoda considered Anakin too old to train, why did he leave Luke hanging around in a desert for so long before training him?

If using Luke against the Emperor was always in his plans, why didn't Yoda just take Luke to Dagobah with him, and train him from childhood?
gpt
Using Luke like an anti-Sith weapon wasn't always the plan, but then fate/the force intervened and forced him to take part in the rebellion. Yoda knew that Vader would eventually find Luke and with zero Jedi guidance at all, he would have easily been turned to the dark size. 

As for why Yoda immediately turned around and seemed to refuse to train Luke when he arrived on Dagobah, the answer is twofold. First, Yoda had to make sure Luke wasn't exactly like his father. If Luke showed the same trouble signs as Anakin: impatience, arrogance, irrational anger at slight inconveniences, Yoda would have walked away. If Luke was too much like his father, Luke was doomed to fall into the same traps that destroyed Anakin. 

Second, Yoda was always a bit of a wildcard and liked messing with people. So his "He is too old" and erratic behavior upon first meeting Luke was both a test and Yoda's actual eccentric personality showing through. Yoda reined himself in when he had to be serious and instruct Luke.