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Lex Jr.: They cast Jesse specifically to appeal to the "young" and "hip" demographic. With Ben (old) and Cavill (receding hairline) we have a film anchored by two mature characters. So they wanted a popular young actor. Jesse has youth appeal and is a box office draw.
I believe you're right in that this was their reasoning, and lots of executives still believe this sort of thing, in spite of zero evidence ever.
But if you're ever in one of those meetings and someone says "We need a young guy so this movie will appeal to 20 somethings" ...just go ahead and scoot your chair subtly away from that guy. He's not smart. His future is bleak, and if you're too close to him, you may get associated with his oncoming string of horrifying failures.
It's the kind of thing that old people who think of kids as some kind of alien species have been doing for decades and no kid has ever asked for. Kids dont have a problem with adult heroes and never have. Yet for decades yellow-tie execs have been trying gimmicks like Superboy, scrappy doo, Indiana Jones has gray hair...bet have Shia follow him around!
An origin story is one thing...young and old alike can find it interesting to see where their heroes came from, but the idea that people under 30 wont watch movies with main characters over 40 is absurd and not based on any example that ever actually happened.
Seriously...apply it to any demo: When was the last time any of you walked away from a movie that you thought might have been good, but didn't want to see because it had no main characters within 5 years of your age? Meanwhile...imagine some of the great classics and just how much they would've been ruined if anyone that dumb had been involved with them. Give Rick a smart aleck teenage busboy in Casablanca...Make Michael Corleone a leather jacket wearing 16 year old in Godfather "They shot my dad!"...honestly I almost threw up a little on my keyboard while typing that.
At some point, these idiots need to let go of the terrible theory that has never been true. Yes sometimes people like movies about their demo...when the movie is about their demo. If its a movie ABOUT high school kids, then yeah...a couple young actors is a good idea. But for an action movie? Anyone care that Bruce Willis wasn't 19 in Die Hard? I think I was just starting high school at the time and I distinctly remember not giving a damn that he was almost my Dads age. The movie was still cool.
bah.../end rant.