Or is it the memory that's the first thing to go? I forget. From personal experience, it's all of the above, and then the knees.
Interesting study. Read the whole thing at New Scientist:
Horvath says that, remarkably, their analysis shows that some parts of the body age at different rates. When they used their algorithm on healthy breast tissue from two groups of women of average age 55 and 60, for example, it churned out a result that was on average two to three years older than the woman's actual age. Whereas across both sexes, heart tissue appeared nine years younger than true age.via HotAir.