Eragon was fifteen, less than a year from manhood. (1.3)
As soon as we meet him, we learn that Eragon is on the young side of maturity. Still, sixteen = manhood? What do you think of that idea? Perhaps one grows up faster when there are Shades and Urgals lurking about…
Mother, aunt, uncle—he had lost them all. (13.5)
As difficult as it is for Eragon to deal with, the death of his family is really what sets him on the road to maturity. For most of us, things aren't so drastic, but it's an inevitable stage in the process of growing up. We have to leave home and forge our own way in the world, much like Eragon does.
You killed them? Saphira sounded surprised.
He nodded. […]
Saphira said gravely, You have grown. (19.8-10)
When Eragon uses his magic to blast the Urgals and defend Brom, it's a sign of his growing maturity. Like it or not, part of growing up for our hero is his ability to use his powers to combat evil.