In my personal experience, the women I've seen become alumnae initiates were actually several years removed from college (all were in their 40s or older).
Admittedly, I haven't had personal experience with a large number of AIs - just reporting the age range for the handful that I have seen.
You mentioned that there is one NPC group that caught your eye due to philanthropy. Is there an active alumnae group for that sorority in your area? That's something you'd want to consider. Most NPC group websites show the locations of alumnae chapters/clubs on the public portion of their websites. I can't speak "officially" for my own group (or for any other NPC group), but I would speculate that the odds of successfully joining as an AI would decrease if there is no alumnae chapter/club in your area. (After all, who would you participate in activities and events with after you joined?
)