Never to old in my opinion, I broke 2 of my 3 terriers to ferrets and they are all over 2 years. The two jrt have worked rat and rabbit, but just did what has been said above every time they made a move or was agressive towards the ferrets correct them, they have to know the ferrets are your and not to touch. My border is just a no go tried everything he just wont take to them. Give it time and put the work in and ATB mate John