" I think that you are getting too hung up on this guy's age and not enough on can he pull it off.
You'd be surprised. The guys who played the "hobbits" in TLOTR were old too.
For example:
Frodo, Elijah Wood, 23
Samwise, Sean Astin, 33
And of course the other hobbits are just as old, if not older. Yet, they looked younger and smaller. "
The hobbits were never portrayed as boys. They were portrayed as what they looked like: little men.
Elijah Wood, who was 18, not 23, when they started filming the Lord of the Rings, was the youngest hobbit and he was portrayed as a young man, which is what he was/is. There was no attempt (and no need) to make the guys playing the hobbits look younger, just smaller.
But Ben Barnes, at 25, is going to have to portray a young boy. This is wrong. Just knowing that he is 25 and watching him interact with Georgie Henley (an 11 year old girl), as if he's around her age, would be very strange and make me very uncomfortable.
Ben Barnes is nearly as old as James McAvoy, Mr. Tumnus, and his interaction with Georgie was slightly unsettling (and rightly so- he was a kidnapper). Imagine how much more bizarre and unnerving it will be to see Barnes with Georgie/Skander.
Again I say, why didn't they just get a boy?
There is separation of ages for a reason...