Fine. You make the movie the way you want to make the movie.I don't think it would be a good idea for him to have any scenes with him in beforehand, because then his character is no longer a surprise, when he appears at the scene to assassinate.
But you DID ask, "Are audiences skeptical in believing in youth
criminal characters?" And that answer is yes, audiences are skeptical
in believing in youth criminal characters unless it's explained to them.
If it works your way, then you will have a better answer. If it doesn't
and your audiences are skeptical then you will make changes in your
next movie.