Just found an egg - 4 chickens only 16 and 19 weeks old!

[Jose21]

Thought my husband and daughter were playing a trick on me - but they insist they weren't. I only put the nest box in the coop today as I read somewhere you should keep it out whilst they are young. I thought they should get used to it. One chicken went in there fairly soon afterwards - but I thought she was just exploring.
Aren't they too young to be laying?
We have four hybrids - 2 we have had since hatching, Warrens, who I know are 16 weeks and 1 day old. We got two others last week - a speckledy and a bluebell, who are apparently 18 weeks old (19 by now). It was the speckledy who went in the nest box. Is it hers?
Sorry to be so clueless - thought I was certain this would not happen soon and even the woman at the farm where we bought the two new ones, told me 24 weeks.... I will try to upload some pictures if I can.
When we cracked the egg (albeit a small one, but with a perfect shell)it had a double yolk.
Any comments welcome!!

 

[Jose21]

This is the chicken I saw in the nest box - 19 weeks old?

This is the other 19 week old - redder in the face

This is the one who squats when I go to her - but she's only 16 weeks old

And this is the other 16 week old

Here's the egg - next to one from my fridge which is on the left

And here's the double yolk

 

[Kaeta44]

I bet that tasted good!

http://muckycluckers.blogspot.com/