In Royal Ancestry (3:359-60), Richardson has her birth as about 1394 (age 28 in 1422 when her mother died), her marriage by agreement in 1396, and that they had four sons, two daughters. The oldest son, Robert was born 1409-13, depending on which age record one accepts. This would make her 15-19 years old at his birth.
The dates, however, seem odd. Her parents were said to have been married before 1372, and they had only three children, so Katherine's birth 22 years later seems strange.
I do not have time to check Richardson's sources, sorry. That may illuminate the issue more.