Depends on what you want to do.
In our case, we wanted a rooster because we wanted a rooster (and for no practical reason whatsoever). We had room in our flock for 1-2 more birds (max), so we didn't really want to go with chicks. We also did a whole lot of research and had many discussions with folks familiar with keeping roosters, so we were well aware of potential pitfalls. Plus, whenever it comes to animal acquisition, we always try to take in an animal that needs a home rather than go to a breeder. So with all of that to consider, we were looking for a mature bird, preferably 1+ years old.
What we ended up with was a 5.5 month old cockerel, but he's a NH and was most definitely beyond the "teenage angst" period and physically matured. He came from a flock with too many roos and needed a home, and was old enough that the previous owner could at least give us an accurate run down of his behavior/habits/attitude. So we knew what we were getting well before we got him, and he's become quite a gem. We love that flippin' bird.
We found him on BYC's Buy~Sell~Trade page. Craigslist and Petfinder are two other really good resources, though.