Which of the following is NOT a sign of aging?

The hardening of the fontanels, which refers to the spaces between the bones of a newborn's skull, is not a sign of aging. Fontanels are present at birth and typically close within the first two years of life as the skull bones fuse together. This process is a normal part of infant development and is not related to the aging process in older adults.

In contrast, signs like wrinkles, gray hair, and decreased hearing are all commonly recognized manifestations of aging. Wrinkles occur due to the loss of collagen and elasticity in the skin over time, while gray hair results from a decrease in melanin production in hair follicles as individuals age. Decreased hearing can result from the natural wear and tear of the auditory system, further compounding the signs of aging. Therefore, the hardening of fontanels is distinctly associated with early childhood development rather than the aging process in adulthood.