i am part korean, and i go by the korean age thing
we have this tradition, [ i went through it ] call the 100th day birthday. and that is your FIRST year. on your 100th day, you wear the traditional dress, and you have a big party, you get hold rings, too. i don't know where mines are, but my mom said she's going to melt them & turn it into a braclet, hahaha.
that's all it is, 100th day is special for the koreans. but, under the reguler system by years, i would be 17. but since i'm part korean, and they have the 100th day bday, i'm 18!