Jamie is Melanie's younger brother, but since both their parents of died, he's more of a son to her. The first part of the book is motivated by Melanie's drive to find Jamie and see if he's still safe.
When we first see him on page, he's "Angry, trying hard to be brave and grown-up, but also showing the fear and pain so clearly in his dark eyes" (17.77). Even without being possessed by an alien, he's still a little bundle of internal conflict. He's not only at that awkward stage between boy and man, but also dealing with every teen boy's worst nightmare: having a sister who's been body snatched!
It's true that Wanda/Melanie's perception of Jamie might be a little skewed. She makes a lot of references to Jamie being a boy or a child, while consistently referring to Jared as a man, like here: "My heart ached after the child, even as [Melanie] longed for me to return my eyes to the man" (27.117). Maybe it's the fact that Melanie raised him. Once a child, always a child.
But we'd like to point out that Jamie consistently acts more mature than Jared, who is almost ten years his senior. Jared is ruled by his emotions, smashing things (like Wanda's face), calling names, and yelling uncontrollably. We never see Jamie lose his cool. Perhaps he's levelheaded and mature because he and Melanie have lived on their own for years, forcing him to grow up fast.