Marta Kristen was born Feb. 26, 1945, so she was 20 on Sept. 15, 1965, when the series premiered.
Her given name is Judith, she is 19 years old and wants to study musical comedy (No Place to Hide).
She gets the first of her numerous hairstyles in The Magic Mirror.
She oversees the hydroponic garden (The Golden Man).
Judy and Penny share a cabin (Kidnapped in Space). It's assumed they originally had their own rooms, but now share because Dr. Smith has taken over one of their cabins.
She helps with the ship's operations (The Haunted Lighthouse).
She will have a descendant also named Judy (Flight Into the Future).