I don't know of any belief that women in their late twenties could no longer bear children.

For historical reference purposes, I'm going to refer to some JA novels, any further discussion of those should go to the Austenations board.

- Charlotte Lucas is 27 when she marries Mr Collins in P&P. By the end of the novel she is pregnant.
- Miss Taylor in _Emma_ has been Emma's governess for 16 years before she marries Mr Weston. Unless she was very very young when she started governessing that puts her somewhere in her thirties. By the end of the novel she has had a baby girl.
- Elizabeth Elliot in _Per_ is 29 but still on the marriage market and merely feels herself to be approaching the years of danger.
- JA was proposed to by a Harris Bigg-Wethers when she was in her late twenties. (From memory he was only 24).