Fuck...why, WHY did this picture make me cry? I mean, all the other pictures I just felt slightly sad but this one picture actually made me tear up and make my heart feel heavy.

Not sure that long life has anything to do with it. I don't see any evidence here that she is old at all.

Hmm, since Reimu doesn't appear to be aged or injured, it could be likely that she is dying or dead of an illness or starvation (being that she looks so frail), playing into the age part, being that Youkai don't age and or often die that easily as humans from those things, and Yukari went to be with or is saying goodbye to her.