The most reliable criteria in Gustafson's method of age estimation is:

Transparency of root is the most reliable among Gustafson's method of age estimation. It is not seen until about 30 years of age. The canals in the dentin are at first wide. With age they are filled by mineral, so that they become invisible and the dentin becomes transparent. Other criterias of Gustafson's method of age estimation: Attrition Paradentosis Secondary dentin Cementum apposition Root resorption Ref: Textbook of Forensic Medicine and Toxicology by K S Narayan Reddy, 27th edition, Page 63.