How old was Christopher Paolini when he published "Eragon"?

He was twenty (20) years of age.

Christopher Paolini is the 20-year-old author of Eragon, a fantasy novel that jumped to third on The New York Times bestseller list just a few weeks after it was published. Christopher's journey to fame began when he was fifteen. He finished high school and was accepted to a college but decided not to go right away. Instead, he set out to write a novel for his own entertainment. Like so many homeschoolers, Christopher tackles projects by educating himself. He read several books on the craft of writing*, and then devised a plot for the "Inheritance Trilogy", three books that would follow the adventures of Eragon, a teenage boy who is befriended by a dragon named Saphira. He spent a year writing Eragon, the first book in the trilogy, and another year editing it.

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Christopher Paolini wrote Eragon when he was just fourteen years old. His parents were so impressed by the manuscript that they decided to self-publish it and take him across America, giving eight-hour talks in medieval gear to promote it. He struggled to get noticed until, eventually, he was spotted during one of these talks by a novelist named Carl Hiaasen. Hiaasen bought the book for his stepson, who fell in love with it. That’s when Hiaasen decided to introduce Paolini and his book to his publisher, Knopf. Knopf published Eragon after a few more minor edits and then it really took off. It remained on the New York Times Children’s Books Best Seller list for 121 weeks and was the second best-selling children’s paperback book of 2005.

https://screenrant.com/eragon-movie-behind-scenes-details-making-trivia/