People with birthdays early in the year goes to school with people from the previous year, so JB although he was born in 94 went to school/was in the same grade as kids born in 93 making him friends with them. Lol so Jackson is only like two months younger and wants to be able to call JB by his name and speak casually, but JB is like no call me hyung, and speak to me with respect (he thinks of himself as friends with people born in 93 not 94)