What will you do once ZUN admits that Marisa and Reimu have been aging all along and that they are about 30-35 years old now.
That's the bottom line. Plus, he does the fandom no favors by doing so. Aging or lack of aging is a pretty big deal and leaving it to the imagination has all benefits and no drawbacks for creativity.
1. Humans age in gensokyo. That's why hermits are a thing in the first place.
2. Between Symposium and Mountain of Faith are confirmed 4 years. Ergo time passes normally in gensokyo
3. Ergo Marisa and Reimu are christmas cakes now
10 years from now:
>Well, Reimu and and Marisa are 45 now and their arthritis is starting to get to them, so the bullet patterns are going to get slower from now on. Also, they're wrinkling. :^)
This is a statement I promise you will never see.
I can imagine that at this point, Marisa is on the border of human and youkai so she probably looks young for her age. Reimu should age though unless she's so hermit-like she turns into a celestial.
The Official timeline state normal years and Sexagenary seasons.
August 12th 2003, 15th of the Month of Shen, 118th season
Embodiment of Scarlet Devil
Season 130 (2015) - Year of Star, Spring and Wood
Autumn - Legacy of Lunatic Kingdom
12 years between EoSD and LoLK. Reimu was at least 12 years old during her PC98 days, so she's around 23 or 24 now.
The canonicity of the PC-98 games is disputed (see Canon for details), however many later games in the series take place near their release dates, but certainly not on them.
Some later works and statements may imply they took place significantly later than their release dates. Cursory pieces of evidence taken together could imply Story of Eastern Wonderland through Mystic Square took place from 2001 to 2003.
Rinnosuke in Curiosities of Lotus Asia Chapter 22's first half believes that Marisa Kirisame only started using star magic after 2001, while Marisa's strategy guide in Story of Eastern Wonderland's omake says her third attack uses star magic, although that is just one attack. However, by the time Phantasmagoria of Dim.Dream takes place, many of her attacks use stars.
Marisa and Reimu are indirectly referred to as "children" in Chapter 22's first half (2001), and Marisa is noted as a child in Lotus Land Story's manual. Although ZUN has stated he doesn't want to give specific ages to those two, the chronology from this becomes more reasonable if Lotus Land Story took place much later than 1998.
Reimu's profile states that Embodiment of Scarlet Devil, which takes place on 2003/08/12, is chronologically not too far from Mystic Square. Also Reimu says in a hidden dialogue that "It's been a while since my last job" in Embodiment of Scarlet Devil.