Biogerontologists have been investigating aging scientifically for over a century, and in that time, they've identified seven primary causes of aging. The last cause was discovered in 1981, and no other causes have been found since, leading many to think that these may be the only ones. In the context of anti-aging research, it has been argued that, if therapies could be devised to remove the toxic effects associated with each of the causes, then lifespan could be extended indefinitely. The seven primary causes of aging, as listed by biogerontologist Aubrey de Grey, are cell loss, nuclear mutations and epimutations, mitochondrial mutations, extracellular junk, intracellular junk, cellular senescence, and extracellular cross-links.
As people grow older, muscular atrophy sets in, partially due to decreased levels of hormones and growth factors. Injecting growth factors or encouraging exercise may somewhat relieve this, although a long-term solution would likely require the use of stem cell therapy.
Due to a variety of causes, the genetic material in the nucleus of cells occasionally mutates, although this cause of aging is kept under control by the massive redundancy of cells in the body. Nuclear mutations are only truly dangerous when they create cancer cells, which can replicate and overtake the entire body. The proposed solution for this problem is to remove the genes for telomerase from all the cells of the body. Telomerase is used by cancer cells to ensure their capability to keep replicating.
The power plants of cells are mitochondria, organelles that produce ATP, the energy currency of all life. Like the nucleus of cells, mitochondria have some of their own genetic material, which is susceptible to mutation. Mutated mitochondria may get out of control and cause serious oxidation damage to tissues of the human body. The proposal for halting this cause of aging is to move mitochondrial DNA into the nucleus, a process that evolution has already been doing for millions of years. The task of bioengineers would merely be to complete it.
There are various kinds of intracellular and extracellular junk that build up in the body, one example being the amyloid plaque found in Alzheimer’s patients. To counteract this cause of aging, scientists propose finding enzymes capable of breaking down the offending molecules without damaging surrounding tissue. The search is currently underway.
The two final causes are cell senescence and extracellular crosslinks. In cell senescence, cells don’t die when they’re supposed to. Instead, these "death-resistant" cells stick around, taking up space and secreting potentially harmful proteins. To take them out requires either injecting something that makes these cells commit suicide while preserving surrounding cells, or stimulating the immune system to target them as they normally would.
Extracellular crosslinks are what causes an old person’s skin to become rough and wrinkled. Cells are normally held together by protein crosslinkages, but these can become too dense and lead to various health problems. The proposed solution is to find compounds (called AGE-breakers) that disintegrate the excess crosslinks.
If all the causes of aging can be addressed using biotechnological solutions, then human beings might be able to live for thousands of years — as long as they do not die from unnatural means.