Free Radicals

  There are many theories about aging, but the ones involving mitochondria stand out amongst the others, especially the “mitochondria theory of aging.” To simply explain it, it has something to do with free radicals, molecules that run out from the ETC and the complexes. Well, when cells copy themselves, they mostly check their DNA, but not the mtDNA and the mitochondria. This way, when you get older, your mitochondria get more and more faulty with the free radicals bouncing around and faulty DNA, so you are less energetic and will eventually die from lack of energy. (Most people die from disease or accidents, though, since dying of "no energy" itself will take more than a human lifespan, but the immune system will become weaker and cancer will eventually destroy the body.)

Basically, when free radicals leak out, they damage stuff inside the mitochondria, and it doesn't just happen after you get old, just that nobody notices it when the damage is fixed/only a little. When people get old, the 'education' center for certain important immune cells that can check for faulty mitochondria/DNA.

Somehow, most birds don't have as much free radical leakage as other animals, and humans can also alter DNA/add measures to prevent free radical leakage.