This is a difficult question to answer because Catherine Howard's exact date of birth is unknown. Catherine's biographer, Lacey Baldwin Smith, writes "almost nothing is known about the early life of Catherine Howard: the date of her birth is open to speculation, her home is unknown, and except for the more lurid details of her childhood which have been preserved in connection with her trial, we know almost nothing about her early friends and environment" - it seems that Catherine Howard is a bit of a mystery!
Bladwin Smith goes on to conclude that she was the youngest of a family of 10 and that she was born before 1525, probably around 1521. A birth date of 1521 would make Catherine around 19 years old when she married Henry VIII.