But that's what I'm saying. Everyone can agree that neither Reimu nor Marisa have really physically changed at all since EoSD except for to match the game's art style (and Reimu's hair growing out over the years I guess). Isn't the change much more likely to be between this supposedly negligible gap?
In fact, hold on a second, I just remembered something. PMiSS refers to an unseen incident known as the "vampire incident," after which spell cards were established. According to the Wiki's timeline, this takes place before or around 1998. Now, like always I question the Wiki's veracity on this date since I don't see any source supporting it, but if this is true then that would completely prove my hypothesis. Thanks to the dates provided by Aya in BAiJR, we know that EoSD takes place in late summer of 2003. So, even if PC-98's dates are unclear, this would establish beyond a shadow of a doubt a large gap between PC-98 and Windows; Mystic Square had no mention of spell cards, so it must have taken place before the vampire incident, which would mean it did indeed take place around the game's original release: 1998.
If this holds (and I really wish I had confirmation on that date), that would strike out Reimu being a teenager entirely. She would absolutely have to be in her twenties by now regardless of what we argue her age in HRtP to have been: If she were 8, then 16 years later she would be 24. If she were 12-13 like I think, she would be 28 or even 29 by now.