volgirl,
Not all NPC sororities do alumna initiation... I wish mine did...
If you're an undergrad, your best option to join a GLO is through formal or informal rush. Your age should be irrelevant ~ if you're a 25-year-old who took some time off before going to college, and you're now a freshman, it's no different than an 18-year-old who went straight through from high school.
If you already have your bachelor's, you should pursue alumna initiation, because you no longer meet the criteria for collegiate membership.
Good luck with whatever you decide to pursue.
