Hmm. Mine started changing gradually, so some were peeping while some were doing a something closer to squawking, and every noise in between. Their 'voices' started changing, I'm guessing about 7 weeks or so. Not sure.
I was just thinking that myself. Its been 20+ years since iv raised chickens. I started up again this year, and I cant remember when mine started clucking. Mine are 5 weeks old and still Peep. I do hear a small change from time to time. Like a boy coming into puberty and his voice cracks.
Mine will be 7 weeks tomorrow and all are still making baby cheeps/peeping noises. All still my itty bitties! I don't want them to grow up because that means they have to go outside in their coop.