In an era of debuts, redebuts and auditions, sometimes it is had to determine who is senior to whom.
The rule of thumb is who debuted FIRST at what time.
Generally speaking, child debuts are not considered to be a regular debut. Some KPop figures were child actors or child performers, but in Kpop their debut as children are not considered to be proper debut.
Huh Solji of EXID had debuted in 2006 in a little known ballad group called 2NB. Although it was an unknown, it appeared on the tv a few times. Still, although 2NB's footprint was tiny, because it had appeared on TV before Solji was considered to have debuted on 2006, which made her Senior to virtually all singers in Kpop when EXID hit the jackpot in 2015.
Sometimes, if a singer is significantly older than the lot, like Park Sojin of Girls' Day, although the person might have debuted later, other singers do not play a Seniority before that person if that person's act is reasonably popular.
Sometimes a singer is with an act for a short time and becomes a member of another act . If the person had appeared on a tv show as a member of the first act, the singer is considered to have debuted at that time. Which is why Hyuna is Senior to every SNSD member since her stint in Wonder Girls is not erased for this purpose.
One can redebut , or be selected in an audition , but in general the rule of the thumb is a person is considered to have debuted when the person first appeared on a TV show as an adult, in Korea or a different country, so the seniority starts then.
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Those who have received Lifetime Achievement Awards from a reputable award outrank everyone in Seniority no matter when they had debuted. First come , first served. So , those who received Lifetime Achievements from major awards, IU (from MAMA, 2014), BTS (from Gaon, 2019) and BoA (from MAMA, 2020), in that order, are considered to be Senior to everyone else regardless of their debut years, and since IU is the first person to receive the Lifetime Achievement Award in the Kpop era, she is considered to be Senior to everyone, including those who debuted before her or are older than her, in KPop, followed by BTS and BoA.