I go by a three to four months after the hen has been laying that gives them time to work out any problems with the egg factory... examples--- Learns to lay eggs in clean nest instead of in the mud. to mind feet when leaving or entering the nest ( had a couple get smashed in the nest ) . and at three to four months after they start to lay their eggs are larger than the first eggs you get for the first month or two. some people may tell you younger, it depends on the breed and the individual hen in question. most of the time it is trial and error I remember it took me two tries to get the egg size right and to get any chicks.
Generally if you want the best hatch rate wait at least a couple weeks after a hen starts laying before collecting eggs to incubate. Pullet eggs tend to be smaller, of odd shapes, and sometimes have major defects. Some will even be missing yolks or whites and obviously those won't hatch. Beyond that it doesn't really matter how old the hen is. Just wait until they've been laying long enough to lay eggs of consistent quality. How long that takes exactly depends on the hen but within weeks to months they should have everything worked out.