I don't think they should cast Jimmy yet. I think they should have someone who looks 16 when they make the sequel to MOS when the Daily Planet plays a big part in the story, and not when Perry White is cast at the 9th hour in an origin story full of military and Kryptonians.
I think Earth One did something smart by making him credible, but I don't think
I also think Jimmy has a tendency to be redundant. You have Clark for the wide eyed idealist, Lois for the biting sarcasm and Perry for the pragmatic yelling. Where does a teenager working on the Daily Planet's website fit in, exactly? He's comic relief. You can take Lois' sarcasm and give it to him, but then what is Lois doing other than whining about not getting her way?
There were good points raised about how he should be a modern kid, and I think that angle, someone who's always plugged in, who's never really listening to anything other than his iPhone, who's obsessed with tech and lives through it, even amidst all this craziness, I think that plays both for comedy as well as some deep thematic elments. When Jimmy takes off his headphones or drops his Droid, it can really put a button on an epic scene.