Yonger the better like four or 5 10-11-12 max
If you watched ROTJ (return of the Jedi) then you would see that when Yoda was training Luke he told him to pull the x-wing out of the swam Luke:"i'll try Yoda really inoyed "no try eitheir you can or can't and he pulls x-wing out of swam.
Luke failed in beliveing in that you can do anything if you know you can do it or you do it can if you belive you can.
And thats another thing why Yoda didn't want to train Luke he was to old when your young you have open mind and belive in what your told to so thats why younglings were trained so young but Luke was old and had lost all his child likeness so it was harder for him to learn he lost trust and hope of what a little/kid have so thats why.
Plus he thought luke would be like Ani has fear and gets to attached to things he tests him he tries to make him discourage and angry to see if he will be the same. But Luke shows detemination ,will but no fear
in the end Yoda agrees to train him Lukes says he's not afraid Yoda: says you will be you will be and hopes he will if he fears dark than he will not join like his dad.