21 is an intelligent enough age, to go around murdering people, and being smart enough to cover your tracks.
If you can convey what you say here in your film then people will
accept it. If you cannot, then people will not accept it. A 21 year
old hit man is not common just like a 12 year old pimp is not common.
In a movie when you choose to do something that is "real" but not
common it needs a bit of explanation. Movies are not real life. If you
show a 12 year old pimp you will need a bit of explanation. And that
explanation cannot be the filmmaker saying, "But there are 12 year
old pimps in real life." If you have a 21 year old hit man you need
to offer some explanation because what you are doing is not common.
It may be "real" but it is unusual.
If the only reason your hit man is 21 is because you couldn't find an
older actor, you will need to offer something to the audience. And
since you cannot be with every, single viewer and say, " 21 is an
intelligent enough age, to go around murdering people, and being
smart enough to cover your tracks." you need to offer some explanation
in your movie.
You know this. I have read posts from you asking "why" this and "why"
that about movies you don't quite understand. That means the filmmaker
didn't do a good enough job and you have to ask. In your movie if you
do not do a good enough job explaining a 21 year old hit man, people
may wonder "why?" You cannot justify it here on the boards - you must
justify it in your movie. Even if YOU know it makes sense and YOU justify
it to yourself, the people watching the movie may need to know why the
hit man is so young. Otherwise they are going to ask themselves, "why?"
In all the examples given - unusual or uncommon or unexpected hit men
there is an explanation so we (the audience) do not have to wonder. We
will buy anything as long as you (the filmmaker) tell us. If you just leave
it - people will wonder.