A good indicator of the passage time is the newspaper's articles written by Aya and Hatate. For example, from them, you can see that
Yoshika appeared on Myouren Temple's graveyard in 2009, but TD proper only happened in 2011 (going by the fact that both
Miko and
Seiga's articles only happen that year).
The games do roughly happen as they're released, which can also be seem from the articles (for instance,
the collapse of the Hakurei Shrine caused by Tenshi happened in 2008, the same year SWR was released).
The games unquestionably happen in chronological order; after all, the events of the games from MoF to TD are directly tied, and even before that, there were references to previous games (the teams in IN [Reimu didn't meet Yukari until PCB, so IN couldn't have happened before it], for instance). The manga also show time passing, and comment on the most recent incidents (like Reimu and Marisa discussign the events of DDC with Sanae in WaHH, and Sanae stating that they hadn't even entered in the previous incident [the events of HM] to Kanako).
Finally, all games happen in different seasons, which's of course another indication of time passing.
As for whether or not Reimu and Marisa age... IIRC ZUN said he doesn't plan on aging them, so even if 20+ years pass they'd probably remain in the same vague "late teens - early twenties" range. However, we
do know that people age in Gensokyo; after all, Akyuu has aged and died nine times and all. It's likely that zunart wouldn't show age differences, anyway.
Although Reimu did seem to have finally grow visible breats in DDC :V