IMO the reason is that Julius Caesar reformed the Roman Calendar into one with 365 days and then Pope Gregory XIII also had a go at the leap years. Without this epic collab our ages in years would be counted differently.
Unless someone is just about to be 18, how would it matter? Even if the calendar is off by a whole month each year, at the age of 15 (our time) you would be 13.75 or 16.25 years old, depending on a missing or additional month.
yes, but you're also disregarding all the shifting going on until the year of your birth. you would have been born closer or further to the end of a year adding or removing up to another year.
This reminds me of an old Mad magazine "Snappy Answers To Stupid Questions".
What do you attribute your long life to?
The fact that I haven't died yet.