Except that some tissues seemed to age far faster than others. "Healthy breast tissue is about two to three years older than the rest of a woman's body," according to Horvath. Cancerous cells aged even more rapidly than that. Tumors appeared accelerate the tissue aging process by 36 years, and healthy breast tissue near breast tumors were an average of 12 years older than tissue elsewhere in the body. In contrast, transforming adult human cells into pluripotent stem cells, which reprograms them to act like embryonic stem cells, effectively "resets the cells' clock to zero," Horvath says.