20. Aging Is Nearly Universal but Depends on Renewability and Damage Control

Life almost universally produces damage and drift, but organismal decline depends on whether the deleteriome accumulates with age faster than it can be diluted, repaired, or replaced. Most organisms age, but some show negligible senescence over observable timescales, and some have life cycles that include partial biological age reduction (rejuvenation) stages alongside aging stages. If organisms have differentiated, nonrenewable cells and structures, they will always age. Aging does not apply in the same way to lineages that proliferate continuously through symmetric cell division, as seen in many unicellular organisms, cancer cells, embryonic stem cells, and induced pluripotent stem cells (individual cells may still accrue damage and then be reset with each division). The boundary between aging and nonaging can also be blurred: the same organism may age or not depending on environment and division mode (e.g., some fungi divide symmetrically and show little aging, but age under stress and switch to asymmetrical division) (2).