I have a small flock of Silkies. Im going through my spring chicks & picking out who stays & who goes. The problem is for every young pullet that stays one of my older hens needs to go. Some of my Silkie hens are 3 to 4 years old but are still beauties. How old are they before they are no longer beneficial to the flock (egg-laying & sitting)? I know that some of the egg producing breeds are only good for a few years, are Silkies the same?