I always thought a pullet was a young hen less than a year old, but older than a chick, but we also call them growers. I found this, I know its American, but similar to what we say, apart from unsexed chicks we call as hatched rather than straight run!
Chickens are classified by sex and age, too. Although these terms tend to vary considerably, depending upon whether they are being applied to exhibitions, or commercial poultry production. Cockerel is a male chicken under one year of age; cock or rooster is a male more than one year old; pullet is a female under one year, and hen is a female over one year of age. When chicks are first hatched, they may be separated by sexes and are then called sexed chicks. If they have not been separated, they are known as straight-run chicks.