Remilia Scarlet DOES have the ability to manipulate the fate of a given subject and enable them to be reborn as a half-youkai. Patchouli herself is practically ageless because she's of the Magician race. It makes sense that the "Elegant and Perfect Maid" of the Scarlet Devil Mansion would be equally exceptional.
Sakuya learned the "eternally 17 years old" tactic from Yukari, no doubt.
I'm in my late teens !I mean, Embodiment of Scarlet Devil was released in 2002 and this year marks its 16th anniversary, so...I'm in my late teens !Eh, but that's what you claimed back in Perfect Memento in Strict Sen--It's obvious that at least 5 years have passed since the scarlet mist inci--Sakuya-san, how old are you now ?I'm in my late teens !