I also sell eggs as a business. We sell hens in their second "laying year." The first laying year is from six months to 18 months, then the second laying year is from 18-30 months. After the first laying year, production goes down about 20%. After the second, you have to take another 20% off. Egg quality also declines quite a bit during the second year and we see some major problems if we keep them into the third laying year. These are production breeds, now--white Leghorns and Red Sex Links. You'll still get fewer eggs and some internal quality problems from heritage birds, but they seem to do better with shell quality even when the hen is older.
I'm with the others, buy your chicks now. We buy our chicks in late February, early March and sell the oldest hens when the pullet eggs have reached full size round about August. It is a pain having to keep two separate flocks. We keep ours on opposite sides of a fence and then combine the flocks at 18 weeks. I don't do it earlier because I don't want my pullets eating layer pellets until at least 18 weeks old.
Every year we buy some, hatch some, sell some, eat some. We also keep detailed records of rates of lay for the flock, and it's helped us out. I'm turning over 85% of the flock this year because we bought some started pullets from Meyer last year and the year before, and now that I have two years of data on them I can see that they're not laying as well as chicks I've had from other hatcheries in the past. That's why I have SO MANY chickens right now--I have enough pullets on hand that I can sell 85% of the flock this fall and not see a drop in egg production.