This article emphasizes the causal relationship in the mechanisms of aging, asserting that among the twelve hallmarks of aging, only telomere shortening is the cause of aging. The “Telomere DNA and ribosomal DNA co-regulation model for cell senescence” suggests that the shortening of telomeres and rDNA arrays can mediate various hallmarks of aging through the P53 pathway. Therefore, the best way to reverse aging and significantly extend lifespan is to increase the length of telomeres and rDNA arrays in adult stem cells within tissues.