Waning attention, executive functioning, memory, and processing speed are merely representatives of general fluid intelligence, the decline of which coincides with neuroendocrine, cerebrovascular, and connectome senescence. Before turning to the consequences of cognitive aging on executive competence, we provide this final chapter within this second part to the end of explaining why cognitive aging begins so early in the human lifespan. This chapter broadly explains why fluid intelligence declines alongside many physical fitness indicators, from respiratory function and cardiac efficiency to strength and speed, and many fertility indicators, from menopause and erectile dysfunction to testosterone and estrogen levels. After drawing parallels between fluid intelligence, physical fitness, and fertility, we conclude with a summary statement showing cognitive decline to be part of a generalized decline in all bodily systems, explicable in the light of life history theory, antagonistic pleiotropy, the disposable soma hypothesis, and other biological theories of senescence.