Here’s a clear age-by-book rundown for Jane Austen, using her birth year of 1775 and the commonly accepted writing/publication dates from her life timeline.

Book When she wrote it Jane Austen’s age
Lady Susan About 1793–1794 About 17–19
Elinor and Marianne About 1795–1796 About 19–21
First Impressions About 1796–1797 About 20–21
Northanger Abbey Original version, called Susan, around 1798–1799; revised later About 22–24 for the original draft
The Watsons About 1804–1805 About 28–30
Mansfield Park About 1811–1813 About 35–37
Emma About 1814–1815 About 38–40
Persuasion About 1815–1816 About 39–40
Sanditon 1817 41

Notes

  • Austen was born on December 16, 1775, and died in 1817 at age 41.
  • Some titles were written in early versions first, then revised much later before publication, so the age can vary depending on whether you mean the first draft or the final version.
  • Her major published novels appeared when she was in her mid-30s to early 40s: Sense and Sensibility in 1811, Pride and Prejudice in 1813, Mansfield Park in 1814, Emma in 1815, and Persuasion and Northanger Abbey together in 1817.

Quick read

If you want the simplest answer: Austen started writing seriously in her late teens, finished some of her best-known novels in her 20s and 30s, and produced her last works in her early 40s.

TL;DR

Jane Austen wrote her books across a long stretch of life: early experimental works as a teenager, the core of her famous novels in her 20s and 30s, and her final fiction at 41.