Again, a funny mistake rather than a joke:
“It is proven that the celebration of birthdays is healthy. Statistics show that those people who celebrate the most birthdays become the oldest”, S. den Hartog, Ph D. Thesis, Universtity of Groningen.
The reason for this error is probably an insufficient grasp of the English language. It’s a literal translation from Dutch. What he meant to say was: “Statistics show that people who celebrate their birthdays more extensively, live longer”. Probably because they are richer and therefore have higher life expectancy (and not because celebrating a birthday once a year radically improves your health – aka omitted variable bias).