If you are in Korea, you will get 1 age or 2 more ages.
Have you heard about Korean age system? It's really, really odd to people who are not korean. Because, we don't grow a year older on our birthday, we do on 1st of January, on new year's day, everybody at the same time!
It can be a little bit difficult to understand. Let me explain with a graph.
Ok, basically, this is it. We count a newborn baby's age as 1. We don't start from 0, like normal age. Then, according to korean age system, we get older when it's new year, doesn't matter when the baby was born, on new year that baby got +1. Then, the birthday come. But, birthday is just the day that the baby was born, and that's it. The baby is still 2 years old. Then, on new year, get older again.
Let's take an extreme example. There's a baby who was born on December 31st, 2014. Since he was born, his age was 1 on December 31st. No matter what, every single people get older on new year, so even though he was just born, he's 2 years old on January 1st, 2015. He literally lived for 2 days but he's age is 2 years old. On 2015, he's always 2 years old. Even on his birthday, December 31st, 2015. And the very next day, he will get +1, which is 3 years old, on January 1st, 2016.
So when foreigners come to Korea, they grow a year older or 2 years. If you already have passed your birthday of that year, you get +1 on your age, and if you haven't, +2.
Since every single people grow a year older on new year, no matter when you were born on certain year, same year born people are all same age. Because of this, we can group people with age. Same year born, same group. This group go to school at the same time, be a same class, and they don't get mixed with other age group. Different age, different class. So normally friends relationships are between same age people. Since we can differentiate people with age, the treatment between different age group has become really important to koreans. If someone is older, we cannot call their name, or 'hey' in korean. It is really rude in Korea. Only to somebody who is same age or younger people, you can call their names or hey. There's some way to call somebody who is older that you.
Upper side is older age, and under is younger age. As I said, it doesn't matter when you call someone who is younger than you. You only have to care about when you call somebody who is older than you. You can call either only that title or name+that title but not allowed to call just name.
You probably know Psy's Gangnam Style. The first lyric is "Oppan gangnam style." He is saying "I'm gangnam style." to some younger girls. Since they have to call older man as "Oppa", he's just calling himself as "oppa" to them.