ah well you can have the original cast but I think the new cast did well in NuTrek off course your opinion may differ.
I thought they did well, too. I just looked at them and judged how they interpreted the roles created by others. I was always aware of Pine, Quinto and Urban as actors trying to capture the spirit of what others have done, unlike when I watch Shatner, Nimoy and Kelley and never think anything other than "Kirk, Spock, and McCoy." That's the difference for me.
Honestly, though, I enjoyed the film and thought most of the cast really nailed it.
How much younger/older would Kirk be? Kirk was was about 30 when he took command of the Enterprise. NuKirk is about 25 when he meets Old Spock. (Pine was in his late twenties). So its well with in the realm of possibilities that Spock could not pinpoint NuKirk's exact age. People age at different rates, but I didn't look that different between the ages of 25 and 30.
I don't recall getting a definitive age of Kirk in the movie. I assumed that he was more like 21 or 22 in his third year at the Academy. Assuming Kirk met Spock upon taking command of the Enterprise (in the original timeline), that would have made Kirk around 32 or 33. Ten years does make a difference. Hell, Shatner changed a lot between
Where No Man Has Gone Before and
Turnabout Intruder. Jimmy Doohan changed even more. That's only three years.
Some people, though, didn't change as much - like De Kelley and George Takei. And boy, I'd love to know how Nero's screwing with the timeline changed Sulu from Japanese to Korean!