Just shy of 32 years of age, the billionth birthsecond could be seen as a sort of coming of age. You're a complete adult now, having typically completed your formal education, embarked upon a career and vocation, fully established, experienced, and capable of assuming leadership while retaining the strength of youth. The moment marking the prime of life. Very close to a hobbit's coming of age at 33 years (ref. The Lord of the Rings).
One could also make an argument for observing one's conceptionsecond (and conceptionday), as that is when one's life really began. We are all about nine months older than our birth age. But it's difficult to determine the exact day, much less the time of day, marking that beginning. Births tend to be more public events than conceptions, and so birthdays and birthseconds are more readily observed.
(I wish I'd thought of this 25 years ago, in time for my own personal odometer event.)