I just rewatched the episode where Terri and Jack's mothers showed up at the apartment, and I can't get over how perfectly cast Mina Kolb was. She does look like Priscilla Barnes, and until I looked her up I thought she was her mother in real life. I haven't seen casting this perfect since Jan Hooks played Mrs. Dubcek's daughter in Third Rock from the Sun.

http://img2.bdbphotos.com/images/orig/v/d/vdp46pglahlep6ld.jpg?skj2io4l

LOL..that was a hilarious scene between the 2 mothers. Jack's mother did great too but Terri's mom looked 10 to 20 years younger!

Mina Kolb (https://en.wikipedia.org/wiki/Mina_Kolb) will actually turn (God willing of course) 100 years old this coming June 7. She is said (https://www.reddit.com/r/threescompany/comments/1tlsd6f/if_the_information_provided_by_imdb_is_accurate/) to be the oldest living guest star on Three's Company, male or female.

WOW ! Had no idea she was still alive. Yes just 2 weeks away from 100. God Bless her.

Now the youngest living (credited actor) Brian Kend (48-49 years old this year ?) from season 2 The Babysitters Ep 17, from 1978. He played interestingly Baby Jonathan.
It appears he is still in the "business" found this possible link. https://rocketreach.co/brian-kend-email_37504013