Why the discomfort when it comes to admitting your age?

So I've observed a fairly common trend on Facebook: people listing their birthday minus the year they were born. Posting it May 22 (for example) rather than May 22, 1986. Why is that, do you think? I guess I find it slightly ironic because usually (maybe it's just me) but often the first thing people want to know when it's your birthday is "How old are you?" You know, what birthday is it that you're celebrating? Your 5th? 15th? 50th? (I'm not sure why the fives, but I'm turning 25 this year, maybe that has something to do with it.)

Anyway, I was noticing that I'm one of only two in my cohort that post the full birthday (including year), and was curious to see if this was representative of a larger population. So I started to look up some of my family and friends. That's right! YOU. You want to know what I found? Well I guess you can look for yourself on Facebook; I found it quite interesting. Maybe I should be more sensitive to people's insecurities about their age. Or maybe they didn't even give it a second thought; just preferred to be a little mysterious. What do you think? Those who post the year, why don't you mind? Those who keep that information private (not to name names, you know who you are--March 11 and November 6, etc.) was it a deliberative choice? Are you trying to keep that information (listing the year would give people all the information they need to figure out your current age, after all) out of public view?

Don't misunderstand. I love you all whether you list your birth year or not (though I admit, I'd silently think to myself "Good for you." and smile when I saw people who listed the full date). Not sure why, I just like birthdays. Always have. It's fun to celebrate life and turn the spotlight on particular people on particular days and say "I'm so glad you were born." Granted, I guess you can do that without knowing specifically what year the person was born (and thus how old they are). Perhaps I'm more the exception than the rule both in feeling fine with sharing my birth year and in wanting to know how old other people are turning on their birthdays. Either way, I'm curious to hear what you think. So comment, people. It would serve dual purposes: clarify the thought-processes driving this intriguing social media phenomenon AND encourage me to keep posting on this blog because there actually are people out there who visit me here:).