Birthday celebrations tend to hold different significance depending on associated changes in our lives. Age 5 may denote the start of school. Thirteen, passage into the exciting and fraught teen years. Sixteen, the freedom of a driver's license. Eighteen, the gateway to adulthood, and 21, the official entry point to the rest of our lives. Then there are the decade markers — 30, 40, 50, and beyond — all linked with change and transformation.

For students, there are the years we graduated from high school and college. (These days, even passage from kindergarten merits recognition!) For PTs and PTAs, there's the age at which we earned licensure. In my own life and those of other entrepreneurs, there's the age at which we launched our own business (In my case, that has occurred a few times, with age 40 being the most notable.) I'm a masters athlete, so every five years brings a new age category in which I'm the youngest, and therefore have a (temporary!) theoretical competitive edge to try to exploit.