Via EurekAlert!: "When the body fights oxidative damage, it calls up a reservist enzyme that protects cells - but only if those cells are relatively young, a study has found. [Biologists] discovered major declines in the availability of an enzyme, known as the Lon protease, as human cells grow older. ... Lon protects the mitochondria - tiny organisms in the cell that convert oxygen into energy. The conversion is never perfect: Some oxygen leaks and combines with other elements to create damaging oxidants. Oxidation is the process behind rust and food spoilage. In the body, oxidation can damage or destroy almost any tissue. Lon removes oxidized proteins from the mitochondria and also plays a vital role in helping to make new mitochondria. ... To fight the oxidant, young cells doubled the size of their Lon army within five hours and maintained it for a day. In some experiments, young cells increased their Lon army as much as seven-fold. Middle-aged cells took a full day to double their Lon army, during which time the cells were exposed to harmful levels of oxidized proteins. Older cells started with a standing Lon army only half as large and showed no statistically significant increase in Lon levels over 24 hours." It is worth noting that the age of individual cells and the age of a person don't have much to do with one another except in some long-lived tissues where the same cells operate throughout life. But you might recall that mitochondrially-targeted antioxidants can increase life span in mice, and mitochondrial damage is important in aging - this research is consistent with all of that, and may lead to another way to extend life via protection of mitochondria.