Might not be from the 60s. She was born in 38 and she looks younger than 22, but she was 25 when Gilligan's Island first aired before her birthday in 64 and I would have guessed she was a bit younger than that so maybe she just looks young for her age.

Edit: She definitely looks a little older than she did in the 1956 Reno High School yearbook.

https://preview.redd.it/suzciwg1rpja1.png?width=243&format=png&auto=webp&v=enabled&s=a5bf7eb7061dee2c7ba6007bea34262c16fe23fb