How old your characters are couches them in a lot of preconceptions about how they’ll act, what their experience base is, and how formidable or capable they may be at the tasks that are thrust upon them in your story and even how they will relate to one another.
Many authors, especially those working on their first novel, tend to create characters who are all about the same age as the author.
This makes some sense insofar as a person can best write about that with which they are most familiar. The drawback is that anyone in your potential readership who falls outside your age range won’t find anyone in your novel to whom they can easily relate. So, unless you are specifically creating your novel for a particular consistent age range, try to mix it up a bit and at least sprinkle your cast with folks noticeably older and younger than yourself.