In the past, my chapter's been open to bidding older undergrads. My pledge class included a married 24 year old, and she was a great VPR.
Still, we once bid and initiated a 25 year old senior. She seemed nice and a great asset, but all that changed after Initiation. She realized that she really didn't have anything in common with her 18-19-20 year old pledge sisters, even accused someone in the chapter of stealing her RITALIN, and a month after she was Initiated, she wrote a letter demanding to go alum. We decided it was best to just cut the strings.
I don't think anyone's heard from her since.
I think that's part of the problem of bidding older women - you never know if they're going to get sick of hanging out with people so much younger than them. Honestly, people can disagree all they want but there's a HUGE difference between a 25 year old woman and a 20 year old. Sometimes it works (there was a chapter that bid a 27 year old and she worked out great for them!) and sometimes it doesn't.
We have a big teaching credential program at our University, so some grad students, getting their credential (a 1 year program) decide to stay active, but most don't. I think if they joined as an upperclassman they are more likely to want to stay active, but if they pledged as freshmen, they usually don't.