You will need to separate them. It is not fair for the younger turtle to be picked on. I am willing to bet the older is a male. It doesn't matter how long they have been housed together once aggression starts you don't have much choice. I would suggest you put him in a tank by himself as chances are he will start attacking your other turtles when this one is removed.

I would seperate them as soon as you can.

Hope this helps
Linda