Age doesnt have anything to do with "sunbaenim" status afaik. That term is about years of experience in the industry or field they work in, i think. For Kpop it seems to be related to who debuted first.
Technically, you could argue that Jimin should be calling Sakura "sunbaenim" she has more experience as an idol and started earlier. Unless there is some rule that foreign experience doesnt count lol, then Sakura's debut date would probably be Izone's date, Oct 2018.