Happy Birthday to you, Pardha and 16,438,356 other people including me. (6 billion / 365, with a naive assumption that there is equal probability for all 365 days.)*
Birthdays like anti-depressants. You want to take the pill before you realize that you are getting older. While its a good thing to realize the eventual 'getting old' thing a.s.a.p., a solar year is too long a time period for any useful introspection. Shrinking time periods like academic year to semester to quarters, quarterly reporting of earnings instead of one annual event, billing hours instead of monthly pay cycles .... are all good evidences. Granularity of time seems to be the fashion and trend. Further, a year is like measuring weight in tonnes. Because, when you measure weight in tonnes, you really don't care about wastage of material in a few tens or hundreds of kgs. All you care about is the number: 25 tonnes.
That was all about birthdays from a self point of view. From an other person perspective, you want lesser granularity to age. You want to be in a certain demographic instead of the exact number, especially when the number starts taking a hike. People think "Well, I am 40-ish", when in reality the age of person is 49! But its going to be painful when I have to choose the 25-32 demographic the next time I fill out a survey or a form. :)
I wish I were a Piraha in matters of couting age. :)
(Pirahas cannot count beyond two and anything more than that would be "many".)