Though I will point out, most of the heroines in Jane Austen books were thought to be confirmed spinsters.
Emma was 21 and said she'd never marry, and she claimed her position allowed her to do so without social implications. She just didn't realize that she was the center of gossip for her choice... Ask Mrs. Elton.
Elinor Dashwood is 20 at the start of Sense and Sensibility, and already thought to be a spinster. Marianne is 17 and it's feared she's on the same spinster path. Both had very negative experiences socially because of their unmarried status.
Anne Elliot, at 26 was a confirmed spinster. She had no social standing, other than that as a companion, "nurse," and future governess. Elizabeth Elliot was widely disgraced because of her unmarried status, and how it encouraged her father to spend on her lavishly, leading to their financial ruin.
When Mr. Collins proposes to Elizabeth, he points out that at her age (21), rejecting him was unwise because it was unlikely that at her age she'd ever be proposed to again. Jane is 23 when Bingley leaves, and part of what is bemoaned by her mother is that he was her last chance before a life as a governess.
So whlie those women did eventually marry in their 20s (though I think Anne was 31 when she married), a very open thread in the book is how all of the above are "past their prime" and most likely destined to be spinsters.
That said, they did remark that how Emma's dad was an old man at 55, and I do believe that in Sense and Sensibility, the father died at 59. The girls mother in Sense and Sensibility was described as an "old matron," and she was 40.