How to determine someone's age

Pull one of their teeth out. Every day, each of our teeth accumulates two lines in its enamel and dentin. In turn, another line shows up every year in the cementum, the stuff that helps keep a tooth in place. Poor diet can change the spacing between lines, but it won't prevent this natural cycle.

This is pretty invasive, though. A new way involving a powerful X-ray synchrotron machine could theoretically be performed on a tooth while the tooth is still in place, but the machine's dose of radiation would be deadly.