I don’t like the idea of having Nimoy and Shatner in the film.
I was hoping for a reimagining of the characters, and a flashback approach prohibits that. The young actors will be straightjacketed into playing…Nimoy and Shatner.
Spock’s only three years older than Kirk. He was born in 2230 and Kirk was born in 2233.
McCoy, being born in 2227, is the oldest of the three.
According to Trek Today, Shatner isn’t in the film, and is upset about it.
But, even that could change.
The link provided by the OP says that Shatner would “likely” appear in the movie.
The Cleveland Plain Dealer suggested Gary Sinise as the young McCoy. I think he could do it. I could see Damon as a young Kirk, too. Probably better to go with (relative or actual) unknowns for both, though.
It’s not clear that McCoy was ever at the Academy. In The Ultimate Computer the Commodore of the opposing fleet in the war games calls Kirk Captain something - a name used at the academy for something now redundant. McCoy never heard of this, and Spock explained it to him.
As a physician McCoy could’ve gone through some kind of direct commision process.
Good point. “Dunsel” is the word. Maybe McCoy went through some kind of Starfleet ROTC, or was directly commissioned as a physician. He’s a staff and not a line officer, presumably (although Doctor’s Orders is a pretty good ST novel about Kirk leaving McCoy in command of the Enterprise, and when a crisis arises while Kirk is off the ship and unreachable, there’s no one with authority to relieve him).
Perhaps they will go with a story in which young Spock is an Ensign, two or three years out of the Academy, and Kirk is a senior Cadet. Does the leaked info specifically say that both are absolutely fresh out of the Academy at the same exact time?
Sir Rhosis
Thanks 
Thank Christ! I was petrified at the idea of Adrien Brody taking the role.