How to identify the age of a "living" person?
The most accurate method known so far may be Horvath's methylation dating algorithm, which uses 353 CpG sites (Genome Biology research article, Nature news article).

From the Nature article, this method was validated with at least one population.


  [After the publication in Genome Biology] Marco Boks at the University Medical Centre Utrecht in the Netherlands applied [the algorithm] to blood samples collected from 96 Dutch veterans of the war in Afghanistan aged between 18 and 53. The correlation between predicted and actual ages was 99.7%, with a median error measured in months.


However, I did not read the original research article and it seems that the dating method works poorly on cancer patients.