Biological aging, a process of accumulating cellular damage and functional loss, begins after sexual maturity. Scientists track aging through DNA methylation, oxidative stress, and telomere shortening. The discovery of the 'immortal jellyfish' in 1988 revealed 'transdifferentiation,' the ability of cells to reprogram, offering a potential model for reversing senescence. While simple animals have abundant stem cells for repair, human cell proliferation is limited to prevent cancer. Some methods like botox, calorie restriction, and ice baths aim to slow aging, but carry risks.