my guess is that he's 1 year younger than Geyol, so 26. Someone else in the comments said the format is year/month/day. Yeseung's ID said: 9Y0329 and Gyeol's is 9X0731. Y comes right after X in the alphabet and therefore, if Gyeol is 27, then Yeseung would be 26.
Wait so he’s not 27 or am I not reading it correctly