In most Western countries, a person is considered 0 years old at birth and turns 1 on their first birthday. This is the system AgeCalc Pro uses by default.
Traditional East Asian age reckoning, historically used in China, Korea, and Vietnam, counts a person as 1 year old at birth, with everyone gaining a year on New Year's Day rather than their birthday.
Knowing these differences is useful when comparing ages across cultures or interpreting historical documents from regions that used alternative systems.