Regarding Gustafson's method of age estimation of adults > 21 years, which of the following is the MOST reliable criteria?

Root transparency is the most reliable of all the criteria in Gustafson's method. The canals in the dentine are wide initially. With advancing age they are filled by minerals and become invisible. This causes the dentine to become transparent. It doesn't occur till about 30 years of age. Also know: If the 3rd molars are fully erupted, it indicates that the person is > 17 years of age. Ref: Textbook of Forensic Medicine and Toxicology by Narayan Reddy, Edition 21, Page - 58