Being open-minded means that we are willing to question everything, including those things we take for granted.
A willingness to question everything, even things we are sure we are right about, can shake us out of complacency and reinvigorate our minds, opening us up to understanding people and perspectives that were alien to us before. This alone is good reason to remain inquisitive, no matter how much experience we have or how old we get. In the Zen tradition, this willingness to question is known as beginner's mind, and it has a way of generating possibilities we couldn't have seen from the point of view of knowing something with certainty. The willingness to question everything doesn't necessarily mean we don't believe in anything at all, and it doesn't mean we have to question every single thing in the world every minute of the day. It just means that we are humble enough to acknowledge how little we actually know about the mysterious universe we call home.
Nearly every revolutionary change in the history of human progress came about because someone questioned some time-honored belief or tradition and in doing so revealed a new truth, a new way of doing things, or a new standard for ethical and moral behavior. Just so, a commitment to staying open and inquisitive in our own individual lives can lead us to new personal revolutions and truths, truths that we will hopefully, for the sake of our growth, remain open to questioning.