A big "maybe." She won't be able to order any alcohol of course.

Some bars are 21+ only. Some are not. Some have one area that is 21+ only (immediately around the bar) and then have a general seating area that accomodates anyone regardless of age; my usual haunt is like this.

Around colleges, for example, most clubs are oriented for the 18 and up crowd. You can enjoy the music, dance, chat with the locals, and I don't think you will have a problem. Same for New Orleans, which draws a huge crowd of college kids who are road-tripping; they'll be more accomodating.

Enjoy your trip!