Carolyn Keene is actually a pseudonym, not a real person. The Nancy Drew mystery series was written by a variety of ghostwriters, including
Mildred Wirt Benson, who started writing in the
1920s and was in her
early 20s when she began contributing to the series.
So, while there's no single "Carolyn Keene" to point to for a specific age, Mildred Wirt Benson was young when she started her prolific writing career, and that's a significant part of the Nancy Drew legacy.