Birthdays are, of course, entirely arbitrary, and not an exact measure of age. As an example: my birthday is July 15. I was not born until three weeks after my due date. And even then, I was only born because my parents and their doctors decided that I needed to be C-sectioned out. (My family jokes that this was just the start of a lifelong pattern of procrastination on my part.) Some of the many other children born on July 15 of that year were born prematurely. That means that I could very well be two or three months older than someone who, for purposes of legal driving, voting, and drinking, is "exactly my age."
But since it's difficult to pinpoint exactly when fertilization occurs unless a baby is conceived in vitro, birthdays are a reasonable shortcut. Good enough for government, right? Age-related laws are so arbitrary to begin with (is a guy on the day of his 21st birthday really more mature than he was on the day before his 21st birthday?) that an additional layer of arbitrariness can't really hurt anything.
Oddly, though, I've found that when I wish a fellow pro-lifer a happy birthday, I feel compelled to say something more than just "happy birthday." Don't get me wrong: I like birthdays. One day a year is designated for your family and friends to let you know how much they appreciate your life. That's awesome. And of course the day you are finally able to see and hold a member of your family is worth celebrating. What's not awesome is when "It's your Xth birthday" is treated as synonymous with "You've been alive for X years." I don't want to appear like I'm making that concession.
And so I say things like this:
I'm also not the only one who does it: