Just for clarification: Mina entered Seoul in her 10th year which makes her age 16 yrs old, however, she stayed back a year thus making her actual 10th year as 17 yrs old, in her 11th year as 18yrs old, and in her 12th year as 19 years old, while Yeji was a year younger compare to Mina.

I hope that clears out the age confusion if ever it will arise.

This story still has not been proofread yet thus errors would be visible.