That's actually a pretty interesting artwork: every final boss is depicted to be approximately the age of their debut game.
So, Chimata is 1 year old, Keiki is 3, Okina is 5, Junko is 7, Shimmy is 9, Miko is 11, Byakuren is 13, Kanako is 15, Eiki is 17, Eirin and Kaguya are 18, Yuyuko is 19, Remilia is 20, and Okuu is crow.

It's final stages by their irl ages!

Poor Reisen, must be hard to be the only young colleague around people that are thousands of years old.

If we're going with final stage bosses, and only count the year, here's a list for the PC-98 games in order:

HRtP: Sariel and Konngara - 27 or 28
SoEW: Mima - 26
PoDD: Yumemi - 26
LSS: Yuuka - 25
MS: Shinki - 25