That usually means they are not any pure breed. When someone has a rooster and some hens of a variety of breeds, and they hatch chicks, it is common to call the chicks "barnyard mix." It just means a mix of whatever breeds in their barnyard happened to mate and lay eggs at the time the person wanted to hatch eggs.they said they were barnyard mix
Some chickens just do get bigger than others. I don't think their size can tell you anything useful about their age.also wondering the age, they said they just started laying this year, but too me they look pretty big like my almost 2yo hens
Some chickens naturally have yellow feet, and some do not.the brown one has very yellow feet which none of my chickens have so not sure what that means.
For chickens that do have yellow feet, what they eat can cause the feet to be more yellow or less yellow. And when a hen lays eggs, her feet become less yellow (because whatever causes the yellow is going into the egg yolks instead.) I've read of people using this to sort good layers from poor layers (more yellow = less laying.)