Volgirl,
There is no technically no age limit for collegiate sorority chapters. However, one thing that has to be considered is whether or not a 25 year old woman is going to enjoy the same things that an 18 year old college freshman does. The reason I mention this is because, should you decide to join a sorority, your pledge class will be made up primarily of teenage college girls whose interests may vary greatly from yours.
Not to say that you shouldn't rush! Being in a sorority is a wonderful experience, and if you truly want to be a collegiate member of a Greek letter organization, then I say go for it. However, if you feel like your interests may differ from the younger sisters, I would suggest following AggieDZ's advice and look into becoming a sister as an alumna initiate where you will interact with women in your age group and above.
Good luck with whatever decision you make! Please keep us updated!
Carrie
