It also varies depending on the school. Some schools are more formal than others.
It always seemed to me that the scientists were the least formal, and genuinely didn't care what they were wearing (old running shoes, baggy khakis, sweatshirts, etc.).
The humanities students can appear a little more carefully bohemian. They'd like to act as though they don't care, but they really do.
Professional schools are usually a bit dressier, but that's really just in comparison to the typical undergrad. It's really more like "adult casual."
B-school is definitely top of the heap in terms of attire. Most b-school students have been making real money for a while, so they can afford to spend more on clothing. They also may have been wearing nice professional clothing, so they might have started to buy their casual clothing at the same stores. However, some of them try too hard and look a little toolish, imho.
If you want to dress a little older, the easiest way is to be more preppy (but not over-the-top). Other styles require a more deft touch; with preppy clothing, all you need is a few polos, a few oxford cloth button-downs, a crewneck shetland sweater for the winter, and a pair of khakis. Look at Lands End and LL Bean if you don't want to spend much. As long as you avoid stuff like cricket sweaters, lobster-embroidered pants, and ribbon belts, nobody will notice you as being anything other than fairly well-dressed.