I don't feel any older now than I did 40 minutes ago... no one ever does, and yet everyone always asks. There are very few monumental birthdays - You've got your 10th (double digit year), your 13th (officially a teenager), your 16th (Come on baby, drive my car!), your 18th (voting, cigs, porn & adulthood), and then your 21st (Pancho's Liquortown!). After that, you are just left with the decade years.
After repeating, "I am twenty-five now... hmmm... twenty-five..." in my head for the past couple of minutes, it does feel a little strange.
I am in my mid-twenties.
I bet that makes my parents feel old. :)