Interesting tidbit from lulu.com:

LITERARY LIFE BEGINS AT 50
– THE BEST AGE TO WRITE A BEST-SELLER, FINDS STUDY
Writers Mature Later Than Scientists Or Musicians, Suggests Research

50 years-old may be a good age for a mid-life crisis but it is the perfect age to publish a novel that you want to become a #1 best-seller, according to a study of best-selling novels and their authors over the past half-century. The average age of writers in the year that their novels topped the hardback fiction section of the New York Times Bestseller List during the half-century from 1955-2004 was 50.5 years – according to a study conducted by Lulu (www.lulu.com), a website for writers and independent publishers.
Over 350 authors saw their novels reach #1 during the 50 years studied; and Lulu researchers were able to establish the age at publication of almost all of them.
Francoise Sagan was the youngest author to top the NYT list – only 20 in 1955 when Bonjour Tristesse reached #1 in 1955; and just 19 when it first appeared in French. JK Rowling was the youngest solo author to hit #1 in the latter 25 years, from 1980-2004 – only 34 when Harry Potter and the Chamber of Secrets hit #1 in 1999, and was soon followed by her next two Potter books.
Agatha Christie was the oldest author to top the list. She would have been 86 when her novel, Sleeping Murder reached #1 in 1976, had she not died earlier that year. James Michener was the oldest in the 25 years from 1980-2004. He was 81 in 1988 when his book, Alaska hit #1.
The most prolific bestsellers over the past 50 years have been Stephen King and Danielle Steel. Stephen King has had 27 #1s since 1979 (averaging 1.04 per year), while Danielle Steel has had 26 since 1984 (1.24/year).
Authors who were nearest to the mean age of 50.5 at the time their book topped the list were Judith Krantz, 50 years 5 months when Scruples reached #1 in 1978, and Joe Klein, who was 50 years 7 months when Primary Colours (published anonymously) topped the list in 1996.
‘A man past 50 should never write a novel’, felt William Thackeray, best-known as the author of Vanity Fair, the novel of English high society from which today’s glossy magazine takes its name. Thackeray published Vanity Fair in 1847, the year he turned 36.
Charles Darwin was 50 when he published his seminal book, The Origin Of Species in 1859 – and all 1,250 copies printed were sold on the first day. James Boswell, the Scottish author, published his famous biography, Life Of Samuel Johnson, in 1791 when he, Boswell, was 50.