I am 12 and 5'7 is a kawasaki kx 250 too powerful for me?

yes and no, I am 14 and have a 04 cr 125 and a 05 banshee i have rode 250's 125's and four wheelers some grown men are scared to ride. You need to ride one first being 5'7 if it is stock height you would be just fine (250 2-stroke is powerful though) I would go ahead and get it , you would grow into it