chick won't stop crying!!!

is it COMPLETELY feathered?  some feather slower than others, and it may in fact be cold.  could you separate a small portion of the brooder w/ new chicks in it and put the one doing the crying in the separate area?  if it is cold, you might also throw an old towel in the dryer, get it good and toasty warm, then put that in with the older chicks.  The warmth may comfort and relax them enough to get them to sleep.

They are probably used to the light and are scared of the dark. I take away lights at about 6 weeks old regardless and mine live outside in the spring where nights get into the 40's still. However, they are acclimated to free raining during the day so any drastic changes will take getting used to. Bringing birds accustomed to 80F to 40F at any age isn't very good.

I had a cry-baby in my one and only time through this, and I suspected it was that I had just taken away their light- I stole my daughters' nightlight from the other bathroom and gave it to the chicks- they went out in just a bit!

Good luck!

I guess mine are just spoiled then.  I keep a light on them until at least 8 weeks.  I do move it farther away so it gives them a bit less heat.  I rarely have 25 chicks at a time (which is the number that the hatcheries use) to be able to produce enough heat to keep warm.  I think they generally do better when they have the heat a bit longer.

I think your older chicks are missing the light being on all day and all night.

I've had my chicks since day 4 (2 weeks now) and I have turned their light off at night.

Maybe I'm lucky because the inside house temp is between 74-76 degrees all day long and my box is away from windows and a/c vents. 

Before turning their light off, I cover the brooder box with a heavy bath towel and a light weight throw blanket. I leave the red light on for about a 1/2 hour more and then turn it off. The will make a lot of racket, but then settle down by the time the light is turned out and sleep the whole night through.

When I wake up in the morning, I turn on the light and about a 1/2 hour later take the covers off... By then, they are all running around ready to eat and have their fresh container of water.

None seem worse for ware and are growing happy and healthy.

Depending on your box set up, you may want to try the blanket method and turn off the light about a half hour after covering them up.