Humans have been celebrating birthdays for nearly as long as we’ve had reliable calendars. The oldest known birthday invitation was discovered as part of the Vindolanda tablets, which are the oldest existing handwritten documents in Britain (you can see images of it and read the original and translated text here, and you really should check it out). The Romans also had a sad custom of waiting several days after their birth to acknowledge and name babies, presumably due to high infant mortality.
We count our ages based on when we were born. It’s acknowledged to be when our lives begin, both legally and socially (except for in cultures like ancient Rome, and I think we can all be glad we didn’t live then). It’s not even an issue in question as far as legal standing goes. The IRS requires that someone actually be born alive to qualify for exemptions. They do allow ongoing exemptions for kidnapped children, even if they are never recovered. The point here is, the U.S. government recognizes that live birth must occur before a being is considered alive. Socially, this is also true, and birthday celebrations and age reckoning are the solid evidence.
No matter what claims you may make to the contrary, you have accepted and actively participated in this system. A system that has existed for thousands of years of human history. And you did it because—whether you want it to be this way or not—it is a truism.
Life begins at birth.