a pre-teen's teeth and a 21 year old's teeth?
http://www.slate.com/articles/news_and_politics/explainer/2009/04/how_old_are_ye_matey.html
One of the best methods for figuring out a person's age is to examine his or her teeth. Every day that we're alive, each of our teeth accrues a pair of tiny lines in its enamel and dentin. Another line shows up every year in the cementum, the substance that helps keep a tooth in place. These features arise from natural, biological cycles; environmental factors like poor diet can affect the spacing between lines, but it won't stop their cyclical recurrence. So to determine someone's age in years, a specialist might count the cementum lines under a microscope, starting at the stress line that shows up at birth in a human's first molar.
http://www.dentalone-md.com/what-teeth-can-tell-us/