William Shatner is four days older than Leonard Nimoy. Why did Spock die at least 8+ years before Kirk?
1966-1931=35 for both actors when they became these characters. In the triad of personalities, Spock clearly represents age, wisdom, and reverence for the calm of logic that comes with it. Kirk is the brash, sometimes uninformed optimism of youth. But there are instances when Spock literally gives up in the face of a no win situation (see Corbomite Maneuver, and The Galileo 7), and Kirk wins with illogical human intuition.
These points are intentional within the story of character development. Late 1960’s Star Trek is above all else a celebration of humanity, built around a core of optimism. Kirk’s gold shirt is literally the ball of light holding things together in the face of the vast, dark unknown.
If you follow both actors’ work, you see that glow much stronger in the one how lived longer, and prospered more.