No, 17 is a great age to start learning to dance. In fact, many of the top ballroom competitors started dancing much younger than that.
Look for a ballroom dance studio or class in your area. You can check the directory on this site, at accessdance.com, google "ballroom dancing" and the name of your town, or check the yellow pages for dance studios. There are also sometimes classes at community centers, YMCAs, etc.
If you live near a college or university, check and see if they have a ballroom dance club. Many colleges do, and some of them accept non-students.
Good luck and happy dancing!