Hens are female turkeys, weighing in at 8 to 16 pounds. Male toms, which are now bred to yield a high proportion of white breast meat that American's prefer, weigh in at 14 to 26 pounds.
The size of the bird does relate to how old it is. Turkeys are labled young at the supermarket because no commercial turkey is older than 26 weeks.
Thanks to modern technology, there is not a significant taste difference with respect to flavor or tenderness between toms or hens.