I think lots of times people don't so much give up on dreams, as outgrow them. What's important to us at one time in life isn't all that important in some other time. Other than that, I suppose lack of money (or unwillingness to take some financial risk) to pursue their dream is probably also a common reason.
I don't necessarily think it's because people get disappointed or jaded. A good, solid, dream can last a lifetime, and not everyone who gets older jaded or allows himself to be affected by disappointment.
For a lot of people, getting older equals personal growth; and personal growth can involve a change of priorities. Then, too, there are people who don't have a dream when they're young and discover they do once they're older and more solid about what they want in life.