"Research indicates that the best estimate of your age is from markers in your cells."
As everyone is different and ages at a different pace, years or any other method might not be as effective in telling someone's age as it is biomarkers. A few examples of biomarkers are the interleukin 6, the transferrin receptor, the insulin-like growth factor, the albumin, the creatinine, and several others.
"The cells examined from the 2,400 people in this study were from blood."
All these biomarkers mentioned above can be found in the blood, in fact, much of what is considered to be a biomarker has to do with blood cells counting and receptors in them as well as biomolecules that are found in the blood. There are many other biomarkers that are measured through the blood such as the total cholesterol, the haemoglobin and white blood cell counts.