I purchased 2 & 3 month olds from a fellow in North GA last year and they started laying at 7 months old. They were perfect hens; lovely in color, egg production, personality, etc. Unfortunately I got 2 roosters out of the bunch, so I traded them. The original fellow wasn't breeding more chickens at the time, so I used a different fellow to trade the 2 roosters out for 2 8 month old hens. The 2 new hens were infected carriers of disease, and bringing them to my flock caused an infection in the entire flock and I had to put all mine down.
Needless to say, I'm a little gunshy now, so this time around I ordered mine from baby chicks and am hoping I can live up to my desire to have a closed flock this time (no new guys), just to prevent bringing in diseased chickens.
I do know folks (and have read of plenty of folks here) who aquire new chickens and add to their flock with no problems though.
Good luck with your decision.