NAD+ (Nicotinamide Adenine Dinucleotide) levels are known to decrease with age in various organisms, including humans. However, quantifying the exact decrease can be somewhat challenging due to variations among individuals and differences in study methodologies. 

Several studies have demonstrated this decline:

1. Mouse Studies: In mouse models, a significant decline in NAD+ levels has been observed in multiple tissues, including the skin, brain, and muscles, as the mice age. For example, some studies have shown that by old age, NAD+ concentrations in certain tissues can drop to half or even less than those found in young mice.

2. Human Studies: Directly measuring NAD+ in humans across a wide age range is more limited than in model organisms. However, some studies have indicated a decline in various human tissues. For instance, a study on human skin found a decrease in NAD+ levels correlating with age. 

3. Mechanisms of Decrease: The decrease in NAD+ levels with age might be due to multiple factors:

    - Increased Consumption: As DNA damage accumulates with age, the body uses more NAD+ to facilitate DNA repair processes, consuming the available stores.

    - Decreased Production: There may also be a reduction in the body's ability to synthesize and recycle NAD+ as we age.

    - Enzymatic Breakdown: Enzymes like CD38, which break down NAD+, increase with age, further depleting available NAD+ levels.

4. Consequences of Decrease: The reduction in NAD+ levels with age has been associated with various age-related conditions, including metabolic disorders, neurodegenerative diseases, and reduced mitochondrial function.

Given the potential importance of NAD+ in aging and age-related diseases, there's substantial interest in therapeutic strategies to boost NAD+ levels in older individuals. This includes supplementation with NAD+ precursors like nicotinamide riboside (NR) or nicotinamide mononucleotide (NMN).

In summary, while it's clear that NAD+ levels generally decrease with age, the exact percentage of reduction can vary based on the tissue, individual factors, and the methods of measurement. It's a topic of ongoing research, and we're still uncovering the full implications of this decline in the context of human health and aging.