There’s a good non-technical overview of cellular senescence, with interviews from some of the field’s luminaries, in the most recent HHMI Bulletin:

Adding just four genes can turn adult cells back into embryonic-like cells, able to develop into any cell type in the body, according to Daley’s studies. In culture dishes, cells from a younger postdoctoral fellow in Daley’s group were “youthful and vigorous,” he says; many of them morphed into stem cells. But Daley’s cells were stubborn, refusing to reverse their clocks. It seems as a person ages, cells get increasingly stuck in their ways.

Daley isn’t taking it too personally. “I’m deficient in a lot of things, and reprogramming seems to be one of them,” he says. He plans to use the observation to understand how to reprogram cells most efficiently.

His finding points out an important concept: cells might not sprout gray hair, get achy joints, or forget where they put their car keys, but they do age. Several HHMI researchers are just beginning to learn what happens to cells as they grow old, and they’re making connections between those changes and cancer, deficiencies in wound healing, and other problems that increase in likelihood as a person ages.