Me thinks the rest of the witches with the exception of Susie age in real time.
Consider the fact Susie has books on how to stay beautiful without it being noticed in this shot >>108168933
From what we have seen in the series so far, Susie has been on the island for as long as the island has existed, so she's probably doing all of this in order to remain in power of the island.
Ramona might have tried to take her place as leader, as she might have realized as she got older, it might not be the smartest thing in the world to let a perpetual teenager in charge.
Would explain why susie is so adamant about things done her way and why she's disgusted at Oscar and Hedgehogs friendship, and tries to keep the ones she has currently by any means necessary.
Truly the deepest yet subtle lore for a show that most probably wouldn't assume that had any.