By the time her character was only 35 she had two grown married daughters. She had to give birth at 15 for the first and 16 for the second.

I think your math's a bit off. In season three Ann freaks out about turning 36. So if we go with the logic that the characters age one year per season, she was 38 when Julie got married (season five) and 41 for Barbara's wedding (season eight).

By the time her character was only 35 she had two grown married daughters. She had to give birth at 15 for the first and 16 for the second.
She got married at 17. She gave birth to Julie at 18 and then gave birth to Barbara at 19.

And yet the title of the thread is that Bonnie Franklin and not Ann Romano was too young.

Were the character and actress the same age?

By the time her character was only 35 she had two grown married daughters. She had to give birth at 15 for the first and 16 for the second.

The origins of the show traceback to actress Whitney Blake who was one of the creators and envisioned the show as a starring vehicle for herself. The whole thing makes a lot more sense when you think of it that way (she was in her late 40's). The casting of Joseph Campanella as the former husband then makes sense. Campanella was in his early 50's when the series started.

I guess you could consider the odd casting of the 31year old Franklin as the mother part of a quirk seen in a lot of Norman Lear productions. Consider the following:

John Amos was 19 years younger than Esther Rolle (who looked old for 50), and only 8 years older than Jimmy Walker.

Sherman Hemsley was 37 when the Jeffersons started Isabel Sanford 58, Hemsley was only 11 years older than Michael who played his son.

Redd Foxx was 49 years old playing Fred Sanford age 65.

John Amos was 19 years younger than Esther Rolle (who looked old for 50), and only 8 years older than Jimmy Walker.Florida (Ester) looked too old for James (John.) JJ (Jimmy) looked young for his age.

Sherman Hemsley was 37 when the Jeffersons started Isabel Sanford 58, Hemsley was only 11 years older than Michael who played his son.
George (Sherman) looked to be 10 years older than his actual age and Louise (Isabelle) looked 10 years younger than her age which was perfect. The two of them together looked to be the same age.

Redd Foxx was 49 years old playing Fred Sanford age 65.
Redd Foxx looked really old for his age. Casting him in the role of a 65 year old was totally believable. He truly looked like he was in his sixties.

she was young! But not immature.

Redd Foxx looked really old for his age. Casting him in the role of a 65 year old was totally believable. He truly looked like he was in his sixties.

and Whitman Mayo was even younger than that...he really looked old too. Interestingly, John Amos played one of Fred's old pool room buddies on an episode too, complete with fake gray hair. He was only 33 at the time.

What's weird and a bit ironic is that when Bonnie Franklin appeared on Gidget (https://img1.bdbphotos.com/images/orig/h/e/hesvzw7n88ypehv8.jpg?skj2io4l) or The Munsters (https://www.sitcomsonline.com/boards/showthread.php?t=75062), she looked awfully old for a teenager.

Do you think she looked old for a teenager here?
https://www.imdb.com/title/tt3496138/?ref_=tt_mv
One other thing, Rifftrax made fun of this short. I'm surprised they didn't throw in an Ann Romano joke there.
https://www.imdb.com/title/tt27009999/?ref_=tt_trv_cnn