Also, ask yourself if you are going to have time with a double major and working. At 25, I was a graduate student and I can tell you that I could SO NOT HAVE handled any other organized activities like being a new member in sorority. Your program may vary.
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"Remember that apathy has no place in our Sorority." - Kelly Jo Karnes, Pi
As has been said, every campus is different. Your campus may be open to transfer and non-traditional students. My campus is very open to military men who have had tours of active duty and then pledge at 20+ years old when they arrive on campus.
You may look 17, but do you connect to 18 year olds? Would you enjoy being part of a new member class with 17-19 year olds? If you think it would be a fit, give it a try.
Typically women have a harder time beyond their sophomore year but, at least you won't have any regrets.