An age in completed years changes on a birthday, not simply when two calendar years differ. Test a birthday that has not yet occurred this year, one that is today, and one that has already passed. Month lengths and leap-day birthdays deserve separate cases because the chosen rule for February 29 can differ by jurisdiction or product.
If the tool accepts times, state the reference date, time zone, and whether it counts full calendar days or elapsed hours. A result near midnight may differ between devices in different zones without either calculation being internally inconsistent. Report the birth date, reference instant, expected rule, and displayed age.
Akousa