Actually Kelley was slightly older than Doohan -- Kelley was born on January 20, 1920, and Doohan on March 3, 1920. Shatner and Nimoy were born four days apart in March 1931, on the 22nd and 26th respectively.
The cast's ages when TOS began in September 1966 (and I'm counting Koenig for comparison purposes) were:
Shatner: 35
Nimoy: 35
Kelley: 46
Doohan: 46
Nichols: 33 (born December '32)
Takei: 29 (born April '37)
Koenig: 30 (born Sept. '36)
And the new cast's ages as of now are:
Pine: 27
Quinto: 30
Urban: 35
Pegg: 37
Saldana: 29
Cho: 35
Yelchin: 18
So while Chekov was originally played by an actor significantly older than the character, now Sulu is.