In the opening credits there a shot of Patty looking like a 10 year old which is similar to the Pajama Party episode BUT it seems all others episodes she is more akin to a mid teenager ?

Whats up with that ?

Well I think at some time in our life, we can look younger than we are. as for Pattys and Cathys age I'm guessing age 16.

Very Diverse Actress in Appearance

My guess is that Patty and Cathy were 15 when the series started. By season three, Patty Duke was starting to look too mature to play a teenage part, especially when she was dressed as conservatively as Cathy did.

The short dark hair (wig?) was filmed way before the show got picked up. Good thing they had sense to do away with it once the first season started! Though, if you look closely in all three seasons there are definite hairpieces worn.

It’s funny that she’d wear a wig AND do advertisements for Breck shampoo!http://https://youtu.be/8EJl_LQ2eOw?si=ryqyo890cB2uyzE1

https://www.youtube.com/watch?v=8EJl_LQ2eOw