Can you tell how old chicks are? Picking?

New to the chicken biz. My 5yo daughter and I purchased 8 chicks last Thursady. 2 RR, 2 Buff Orp, 2 Slilver laced wyandote, 2 domoors. (names may be spelled wrong, spelling is not my best subject) The silver laced seem to be smaller that the rest. These chicks have really grown in 6 days. The feed store did tell me that the sliver laced were younger. The others are getting tail feathers about 1/2" long. Is there a way of telling how old they are by their feathers or any other way?

I also notice today when I was cleaning the cage that one of the buff orps looked to have some blood just above the tail feathers on the back. it looked like it may have been picked. What should I do in this case? Should I move her somewhere by herself? Does this sound like picking?