A pullet is a young female chicken. Some believe it is one not yet laying but it is actually a female under 1 year of age (like a filly is to horses)
And any young male chicken under the age of 1 year is a cockerel, not a rooster. Pullets become hens when they turn 1 year old.
According to definition a young hen less than a year old. I have always gone with a young hen in her first season of egg laying, basically the same definition. The younger a hen is the more eggs they lay, with each passing year they slow down a little on egg production. A five year old hen will lay far fewer eggs than a "pullet". Not only does a pullet lay more eggs they are more efficient at converting feed into eggs so less food to create the same number of eggs.