Ages of the characters in "The Deadly Years" based on the canonical citations by Christopher
and the assumption that the year in the episode was 2267. Their ages in 2258 (the time of ST09) are in parentheses:
-- Kirk, b. 2333, 34 (25).
-- McCoy, b. 2227, 40 (31).
-- Scotty, b. 2222, 45 (36).
-- Chekov, b. 2245, 22 (13).
The ages of Spock, Uhura, and Sulu are open to speculation. Shatner and Nimoy are the same age, but most think Spock is three years older than Kirk. Nichols is only about a year and nine months younger than Shatner and Nimoy, though it's likely Uhura was younger than that realtive to Kirk. Takei is about six years younger than Shatner, so that would make Sulu 28 when Kirk was 34, even though Sulu's birthdate on Memory Alpha is 2237.
All of the actors playing those parts in ST09 were older than the characters were in 2258, but not so far off that they couldn't play those ages convincingly.
As has been noted many times on this board, Chekov's character is the only one with a rather large age discrepency across TOS and ST09. He says he's 17 in the movie. He "should" be 13. Call the change in his age artistic license to get him on board with the other six more quickly than it happened in TOS.