NAD+ levels drop steeply with age. Direct measurements in human tissue found levels of 8.5 units at birth, 2.7 units in young adults, and just 1.1 units in people over 71 — an 87% decline over a lifetime.
Source: Massudi et al., "Age-Associated Changes in Oxidative Stress and NAD+ Metabolism in Human Tissue," PLoS ONE (2012); Nakahata & Bessho, "The Circadian NAD+ Metabolism," BioMed Research International (2016)












