Too many cigarettes and too much sun2000 on pale Irish skin undoubtedly led to her looking older. Also, throw in the stress from a difficult divorce and her depression issues as other reasons why her youthful appearance prematurely diminished.
She was still a very attractive woman at age 34 (her age when she made that Twilight Zone episode), but she did look significantly older than her real age.
I wonder what she looked like prior to her death, but so far I've never seen a photo of her taken after she walked away from acting in the mid 60's. Did the people she worked for and with as a temp typist in the 70's know that she had been an Academy Award nominated actress who had acted alongside Bill Holden and David Niven? Or had her appearance changed so much that no one recognized her?