You should be able to tell if they are old, I think. They get pretty ratty looking when they age. At 3-5 months old they should look pristine and a little smaller than full grown. That is a little young to be laying. I have one young rooster that is only about 10 weeks old strangling out crows already and he is pretty small. Check for mites is what they always say but I am no sure I would see them. Perhaps sprinkle them with diotomateous earth just as a precaution. Are you adding them to your flock? I always assume people are honest. After all you are going to their house.
What I'm hoping for is that the eggs will still be small, indicating they just started laying. I've also had hens lay before 6 months. He said some had started laying not all.
Woman was very nice and said she had selected a variety of color for me. They are beautiful healthy looking birds. They were the last ones she had and another caller kept wanting them but she had already promised them to me. I was able to lower the price from $120 to $110 by whining that I had come from a long way. 
She also showed me the 4 pullet size eggs that they had laid that morning. I like the fact that they were different shades of brown. Some were dark while others were lighter.
