Aside from the fact that they're legal adults...
Greek life isn't for everyone. I don't think anyone should be forced into it or made to feel like they HAVE to join a chapter to get the most out of college.
That being said, I would say social rejection is difficult at any age.
Maturity plays into it, but not everyone wants to compete socially. Some people have social anxiety, others are introverted and dislike being part of a crowd, and a lot of people are happy just studying and occasionally hanging out. The dues, all of the social obligations...I wouldn't judge someone for not going Greek. I think it's good for most people to rush, but part of the process is deciding "Okay, not for me."
I'd also rather have someone drop out if their heart's not in it than potentially "take a bid" from someone who wants more than anything to be Greek.