How? Because the first cell that became Leonard Nimoy appeared in this galaxy in the year 1905. What so special about 1905? That was the year his mother (Dora Spinner 1905-1987) was born, subsequently the first distinct cell of Spock, oops I mean Leonard Nimoy, to exist appeared under President Teddy Roosevelt.
While William Shatner will turn 106 due to the same reasoning. The first cell that became the corporeal body of the destined to be famous Captain Kirk, formed in this solar system in the year 1908. Although this cell (egg) would have to wait 23 years when the other half joined it and became William Shatner. Thus Shatner was born under Canadian Prime Minister Wilfrid Laurier.
The groovy twist is that by conventional means William Shatner is older than Leonary Nimoy by four days, since they were both born in 1931. However measured from a cellular level, Nimoy is three years older than Shatner!
How is that for a fun Star Trek Fact!!!
Sounds like science fiction? Nope, it's science fact!
But every once and a while, using the cellular point of view, it happens every so often that a father is younger than his children.
How? The simple rule is: if you are a man and your mother is younger than your wife, then all your children from your older than mom wife are older than you, on a cellular level.