That's an opinion.As a writer, letting yourself be guided by the characters to discover the story yourself as you write, it's the most exciting and fulfilling way to work.
And it does lead to problems like opening up too many subplots and writing yourself into a corner because there's no succinct way to resolve all of those B and C plots.
It also leads to a problem where your characters start hitting Sitcom syndrome because, 'what if X did Y?' starts becoming an excuse for advancing the plot.
Paolini was also a teenager when he started writing those books.Eragon left open most arcs, the author was unable to close them even despite making another book, that wasn't originally planned, to try to do exactly that.