Calculating exact age is more complicated than simply subtracting your birth year from the current year. To get the precision down to the days, you have to account for varying month lengths and leap years.
The Logic
First, subtract the birth day from the calculation day. If the result is negative, it means a full month hasn't passed yet. We borrow a month, and add the number of days of the previous month to the calculation day.
Next, we subtract the birth month from the calculation month. If the result is negative, we borrow a year (12 months).
Finally, we subtract the birth year from the calculation year.
This ensures mathematical accuracy across leap years and months ending in 28, 29, 30, or 31 days.