The problem is that as the mitochondria become damaged so to does the integrity of their electron transport machinery and free radical leakage grows ever worse, raining havoc not only on the mitochondria but on the cell at large. The end result - run down, mitochondria that produce less energy and degenerating tissues. You'd have to admit those sound like the hallmarks of ageing. Many aging diseases and symptoms fall easily into this picture. Genes, proteins and lipids are vulnerable targets of free radicals.
Cancers result when key control genes are hit.
Heart disease may result from an attack on low density lipoproteins and wrinkling is probably the result of hits to our collagen proteins. These include enzymes called SOD and catalase as well as nutrients like Vitamin E and beta carotene. With ageing, the levels of free radicals rise overwhelming the anti-oxidant defences. So the mitochondria, and the free radicals they produce during metabolism is emerging as the central theme in the aging story. But birds are an interesting exception.