The method
First count the full years since the birth date. Then the remaining full months. Then the remaining days. That is why two people born a few days apart can have the same age in years, but not the same exact age.
Months and leap years
Months have 28 to 31 days, and leap years add a February 29 about every four years. A good calculation uses calendar dates, not an average of 30 days per month. The calculator does exactly that.
An example
Born on January 1, 2000, you are 26 years, 9 months and 23 days old on October 24, 2026, and your next birthday falls on January 1, 2027, 69 days later.
Why exact age matters
For a birthday, a form, or simply out of curiosity: age in years alone rounds things off. Exact age says where you really are, between two birthdays.