Once again I think this is one of those things that ENTIRELY depends on the campus. We had a sister (undergrad) who pledged when she was 25 - she had been in the service and such, and wanted to do this. Her big LOVES her, and I don't think anyone ever had a prob with her being older. I know some of the fraternities had older guys active. Now if we would be at a school like LSU where 19 year olds at rush are considered somewhat long in the tooth (LOL) it would be another story. I think even juniors might feel somewhat uncomfy in that type of situation as the houses are so big you're lumped with your pledge class all the time.
A lot of grad students come from schools that didn't have Greek life, they didn't have time, or their parents wouldn't allow them to pledge. Once they are 21+ they can do whatever they want of course. I think the reason for grad students not being involved is not so much age, but lack of time.
Never ever say "I should have"!
