Dianne is right. It does depend on your school.

However, I was 20 when I pledged and my age didn't matter. In fact, one of my pledge sisters was 23 and married when we were initiated.

For more information, please check out this thread:
http://www.greekchat.com/forums/ubb/...ML/000291.html

I bet that there will be many people this Fall who are wondering the same thing you are!