I got that age verification thing too and it was saying some of my info was incorrect. I clicked, I am a korean citizen. and it let me go. So as far as YouTube is concerned I'm a Korean now.
Some of the verification systems use your ID number to verify, so DOB and nationality both come into play. They ask for the first 7 digits of your ARC, and USUALLY (but not always) if you are a Korean citizen. Some of these systems fail because they only check if 7th digit is a 1 or 2.
It is intersting to see what will happen when those born in 2000 are considered "adults". It will be like Y2K all over again!
The value of the 7th digit depends on gender, century born, and nationality.
Korean born in 1900's: 1 for male, 2 for female
Korean born in 2000's: 3/4
Waygookin born in 1900's: 5/6
Waygookin born in 2000's: 7/8