One thing I've been noticed recently is how people regard age and "rank" or "position" as the years go by. When we're born, people refer to babies' age in matters of hours, days, weeks, and then months. This goes on through the first year and a half or so.
"How old is he?"
"He's 18 months."
Once kids can talk, they continue tracking their age fractionally through high school. Half years count, sometimes even smaller fractions. Countdowns to the next mark are important.
"I'm 15 and a half."
"I'll be 17 in four weeks and three days."
Once a school year is over, students immediately promote themselves to the next grade. As soon as the bell rings at the end of the last day of school, juniors declare themselves seniors and seniors declare themselves graduates.
However, beyond this point and especially beyond college age, years start to become measured in wholes. We start to hold on to the a current age number all the way up to the final day before a birthday. If people are inclined to lie about their age, it's more likely done to be younger rather than older as would be more common for those under 21 or 18. The 10 year marks at 30, 40, 50, etc. can, for some, become the subject of trepidation or ridicule.
I haven't really drawn any conclusions about this phenomenon. My attention has just been called to it from observing those younger than me celebrating their age while those my age or older disregard or even avoid it. Having frequent contact with the former group simply serves to heighten my perception of the contrast.