Among the secondary changes in tooth the most useful one for age determination is:

Root transparency is not seen until about 30 years of age. The canals in the dentin are first wide. With age they are filled by mineral, so that they become invisible and the dentin becomes transparent. It is more reliable of all criteria by Gustafson's method. Factors that determine age by assessing dental tissues in Gustafson's method: Attrition Paradentosis Secondary dentin Cementum apposition Root resorption Transparency of root Ref: The essentials of forensic medicine and toxicology by Dr K S Narayan Reddy, 27th edition, Page 63.