The traditional Korean age (세는 나이, 햇수 나이) works this way. When you're born, you're immediately one year old (!). Afterwards, you just add one on the 1st of January each year. So if you're born on November 1st, you become two years old two months later. In "만 나이" which is the same as western way of counting age, you are not even one year old by that time, so there can be a difference of two years. This traditional system is not just Korea's. China, Japan, and other Asian countries used to use this system, but every country has dropped it for obvious reasons. In Korea, even though 만 나이 is the only officially recognized way, people still commonly use the 햇수 나이 in daily life.