Pick people who can actually play the part not people who are too old or too young
Superman - Gerard Butler
The huge, muscular body of Leonidas from 300, coupled with the down to earth, likeable personality of Milo from Bounty Hunter or Stoick from How To Train Your Dragon. Butler can be friendly and loyal, delivering the part of the Boy Scout, while still appearing massive and dwarfing his fellow teammates in the way that Superman needs to.
Batman - Jason Statham
Here me out on this one. Forget that he's bald; he's going to be wearing the mask through the whole movie. As a Justice League movie, he doesn't have to be a convincing Bruce Wayne, just a convincing Batman. Statham is tough and in shape enough to carry himself in a fight, but small and agile enough that his fighting style will lend itself more toward the acrobatic, which will further contrast the size difference between him and Batman. On top of that, he has a strong jaw, with mild signs of aging, and a naturally rough and gravelly voice, so he can play the role of Batman without having to do some rediculous fake voice like Bale did.
Wonder Woman - Ali Larter
Look at some of her more muscular parts played, such as her role on Heroes or as Claire in the Resident Evil movies. Give her dark hair, like she sported in Final Destination or Afterlife. Strong jaw, high cheekbones, beautiful but in a tough-girl way. And she has no problems playing a role where she is to be a bit more bossy and demanding. The perfect choice for an Amazon Warrior Princess.
Green Arrow - Ben Foster
If you want a wirey muscular hothead with wild blonde hair and a scruffy Robin Hood goatee, sporting a bad attitude and a chip on his shoulder, look no further. It's a role Foster has already played a half dozen times and done exceptionally well. Ben Foster might not make the ideal Mayor Queen or CEO of QueenCorp, but when it comes to sporting the green, shooting the bow, and being the one to freak out and take everything more personal, he couldn't be more perfect. Foster brings the raw emotion to the team that otherwise is filled with stoic characters, eternally patient and full of self-control. He balances the cast excellantly, and gives Batman the polar opposite for him to butt heads with.
Aquaman - Jai Courtney
Blonde curly locks, firm square jaw, stacked with muscle, but with a personality that means no one else can entirely take him seriously, even if he himself fails to see the joke. Watch his performance for 10 minutes in the first season of Spartacus and you will see Aquaman in action; all he needs is a trident and an excuse to claim that he's royalty.
Martian Manhunter - Peter Mensah
He has the perfect shape of head, the refined intelligent sounding voice, and a muscular enough physique to wear only the double bandoliers and proudly show off his chest. Mensah is slender enough to not overshadow Butler as Superman, but still buff enough to stand equal to Courtney's Aquaman and tower over Statham's Batman. He has also proven to be adept in a number of accents, which could likely be utilized to give his words that slightly foreign, exotic feel of being from another planet and while still seeming intelligent, using English not entirely as his first language.
Green Lantern (Stewart) - The Rock
Similar to his role as Sarge in Doom, Johnson can be the straight laced military man. Johnson has a flexibility as an actor though that allows him to dip into some comedic acting, particularly the type of comedy where he takes himself seriously through behavior that the audience cannot. In particular, if Johnson could be granted the chance to interact with a brief cameo of Bruce Campbell playing Guy Gardner, with John Stewart trying to give him military style orders, I'm sure we would see endless laughs.