Adding a new chicken to a relatively young flock

[kenyabob]

I have recently dumped a rooster and my flock feels one short (obviously). The chickens have gotten to a pretty good size and around four some months now. They haven't laid anything yet, but I can tell it's any day now. In regard to augmenting my flock - would it be bad to get a bird around a year old? I have fears about getting a bird much younger than them, but I am unsure if the same applies to one six or so months older than they are. I am looking around on craigslist for various hens and I was wondering what people's advice was about adding to a young flock.