1. Older adults are less open. I wouldn't just start texting someone from work about my favorite bands after knowing them for a couple weeks. In college that was common.

2. Children and family responsibilities are primary relationships, while in college friends are primary relationships.

3. Energy levels drop and responsibility rises as you age; limiting what you want to do.

4. There is a light social pressure to "act your age" that has a small governing influence to adult relationships.

I have good friendships, but almost all of my adult friendships are from college and highschool. Hanging out is usually a set activity like hopping on a game, grabbing lunch, or maybe biking, golf, etc on a sunday. Work is where I meet people and all of those relationships tend to be tainted by the professional relationship.