Alex Rider ain't no Harry Potter—there is no prophecy foretelling his rise, no mystical help from on high, no wondrous academy to enroll in. Sure, his uncle has been preparing him to be spy without his knowledge, but a few karate classes aren't going to prepare you for what Alex goes through over the course of Stormbreaker. Ultimately, Alex is only able to get through his trials because he is able to adapt, growing into a man in the process.
Alex isn't very different than you were (or will be) at fourteen. He lives a normal life, and he loves to "play soccer" (4.81) with his friends; he's mostly disinterested in school, although he can be focused and engaged with subjects that he enjoys. He doesn't have any special skills, and there are no tricks up his sleeves. James Bond he is not. Or, not yet, at least.
One monumental event changes everything, though. Ian's death has many practical implications for Alex, from threatening his financial security to almost forcing him to move into "an institution" that's "not a very pleasant place" (4.89). Equally important, however, are the changes within Alex sparked by the tragedy. We see Alex, though sad beyond belief, using all of his abilities to discover the "truth […] about his uncle's past" (1.55). Already, we can see this distracted young boy take his first steps into adulthood.
The next steps—which are more like leaps—come in the form of Alex's burgeoning relationship with MI6. Mr. Blunt and Mrs. Jones treat Alex with a mixture of respect and ambivalence. On one hand, they respect the kid for his spunk and believe him to be "extraordinarily brave and resourceful" (4.73). On the other, though, they treat his life as expendable, although it must be mentioned that they seem to think that all life is expendable. Regardless, they are the first people to believe in Alex's immense potential.
If MI6 is a leap into adulthood, then consider boot camp to be the inevitable free fall. Alex is tested more than ever before and can even sense the "taste of defeat" (5.3) at times. His experience there centers on earning the respect of the older men in his group, all of whom "taunt or humiliate him" with the (un)affectionate nickname "Double O Nothing" (5.22-23). But Alex eventually earns their respect by his adherence to the bro code, becoming not a trainee, but a peer—a.k.a. man.
Training is one thing, but it's a whole different game out there in the real world, so although Alex grows a lot in the time before his excursion to Sayle Enterprises, it's nothing compared to the growth he experiences during the mission. Alex's life is put in danger repeatedly and, over the course of only a few days, he wins more no-win situations than we can count. How does he do it? Two things: his natural grit and determination, and the memory of how his uncle "never stopped" (11.59) fighting.
By the end of the novel, Alex has taken control of his own destiny. Remember: At the start of things, he is just a fourteen-year-old boy with death in his past and uncertainty in his future. Yassen is right when he tells Alex to "go back to school" because "killing is for grown-up and you're still a child" (17.78), but we've seen firsthand how grown-up Alex can be. He might not be a man quite yet, but he's well on his way.