The paper is called DNA methylation age and human tissues and cell types and it's from Genome Biology. Here is a Nature article based on the paper.
I have submitted this to LW because of its relevance to the measurement of aging and, hence, to life extension. Here is a bit from the Nature piece:
"Ageing is a major health problem, and interestingly there are really no objective measures of aging, other than a verified birth date," says Darryl Shibata, a pathologist at the University of Southern California in Los Angeles. "Studies like this one provide important new efforts to increase the rigour of human aging studies."
Note: The discrepancy in spelling ("ageing" vs. "aging") is in the original.