Ideally the date distilled, but that might be difficult to find. My second choice would be to get a whisky distilled the same year that they where born.
I plan to get a bottle of whisky from the year my son was born for his 20th birthday (leagel drinking age for spirits in Norway) and a vintage port from the year he was born for the day he gets married.
And if he never marries? I will drink the port myself. Then I'll be happy either way!