A person's growth as a person does not end when they come of age. I am a very different person now (at 53) than I was at 21, to the point that I am abjectly ashamed of my younger self.
It is true that when a person comes of age, they no longer have to deal with the struggles of a person who is coming of age. Once you've gotten the girl or guy of your dreams (a common ambition in coming-of-age tales), you no longer need to get her or him; you move on to weightier things, for which the earlier struggles prepare you.
We also find (at times) that the steps we took to overcome a difficulty in an earlier stage turn out to be less effective than we had hoped, or—even worse—have compromised our ability to deal with a more important problem later. What seemed at the time to have been a wise and reasonable expedient turns out to be a fatal error, and the hero must learn to stop doing what is easy and start doing what is right.
It's difficult for us to appreciate it now, but once upon a time sequels were rare in speculative fiction. Galatic Patrol, the first novel in the acclaimed Lensmen series, ends with <PERSON> decisively beating <PERSON>, the head of a galactic criminal/military empire. 999
It was a surprise to many readers when in the opening chapter of Gray Lensman, we discover that <PERSON> was only a minor flunky in a larger organization. (The <PERSON> series is also an arms race gone mad, but that's another topic.)
Addendum
You can follow up the coming-of-age novel, and take the character's growth and path in a new direction, by constructing a Hero's Journey in which the coming-of-age portion you've already written becomes the mere beginning of the Journey. You can do this even if the first volume is a complete Hero's Journey in its own right, but you have to be very sure that this time, the features and challenges of this journey are different in kind and not merely in degree:
* If the struggles of the first volume are met by being clever, in the next stage the struggles are such that no humanly-possible degree of cleverness will suffice; instead the Hero must grow in a virtue that is more fundamentally important than cleverness and perhaps is even antithetical to it (such as honesty or humility).
* The dangers are not merely physical, and the struggle is not limited to the physical realm. The ideas of what constitutes good and evil were accepted without question in the first stage; now they need to be checked. The Hero's being on the right side of the struggle can no longer be taken for granted, but instead is something that has to be held to a real test of truth.
* Friends become enemies and enemies become friends. 627