Horvath’s ageing “clock” relies on measuring subtle chemical changes, in which methyl compounds attach or detach from the genome without altering the underlying code of our DNA. 

His team previously found that methyl levels at 353 specific sites on the genome rise and fall according to a very specific pattern as we age - and that the pattern is consistent across the population. The latest study, based on an analysis of blood samples from 13,000 people, showed that some people are propelled along life’s biological tramlines much quicker than others – regardless of lifestyle.

This is not the first time that scientists have observed so-called epigenetic changes to the genome with age, but previously these were put down to wear-and-tear brought about by environmental factors, rather than indicating the ticking of an internal biological clock.

Wolf Reik, a professor of epigenetics at the University of Cambridge who was not involved in the work, said: “It now looks like you get a clock given to you when you’re young. It gets wound up and the pace it’s ticking at is dictated by this epigenetic machinery.”

“I’m sure insurance companies are already quite interested in this kind of thing,” he added.

Horvath said he has no plans to market the test, which costs around $300 per sample in his lab, but admits he has run his own blood through the analysis.

Some questions:

1. Once we understand the clock can we reprogram it?

2. Even if we cannot reprogram it would most people want to know if they're aging quicker?

1) Hope so. 2)people can probably guess who is a quick ager versus others at a certain point 

The study also raises more questions:

Is death caused by epigenetic changes associated with chronological aging? Or does it merely spur diseases? What is the real role of epigenetic changes in aging or death?

Source: http://futurism.com/genetics-breakthrough-could-one-day-help-slow-the-aging-process/