Actually, if we want to clarify terminology, which I agree is important a pullet is a pullet until she's a year old then she becomes a hen. A cockerel becomes a cock at one year of age. Rooster is an imprecise term that apparently means any male.
To add confusion to definitions of pullet vs hen and cockerel vs cock. Working definition varies with type of interest in the birds. I am a biologist coming from a gamefowl background. Those interested in production and show have need for different set of criteria.
Criteria that can used to distinguish pullet from hen
1) Point-of-Lay which is biologically significant with respect to sexual maturity
2) One year post-hatch which is arbitrary but is easy for record keeping
3) Biologically significant and based upon completion of second adult feather set when full maturation of slow growing breeds like games occurs which occurs between 12 and 18 months post hatch depending on hatch date.
Criteria that can used to distinguish cockerel / stag from cock
1) Biologically significant with respect to sexual maturity and completion of first adult feather set (the
not very good definition I used to in first post)
Bullstag is interval between completion of first and second adult feather sets
2) One year post-hatch which is arbitrary but is easy for record keeping (bracketed by bullstag interval)
3) Biologically significant and based upon completion of second adult feather set when full maturation of slow growing breeds like games occurs which occurs between 12 and 18 months post-hatch depending on hatch date. This is also very important for Jungle Fowl which are sexually mature for 6 months to a year before becoming a harem master. Second and subsequent feather sets of such adults are distinguishable in some breeds (especially games). Length of spurs can also be a criterian.
Options 1 and 3 are most informative in understanding the physiology and behavior of both genders.