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That sounds like a plausible reason for evolution to have given us lower NAD levels when we're older.

For the vast majority of our species' existence, we overwhelmingly died of causes other than cancer, so if there is any evolutionary pressure in this regard, it should be weak. Cancer as a significant cause of death is a Modern Age phenomenon.

Also, the mechanism may be more complicated. What if NAD+ levels influence the immune system somehow? (Our first line defence against cancer.) Metabolism isn't a straightforward (A -> B) system at all.

I have to disagree with you on that. There are a great many mechanisms we evolved, most long before we were human, that appear to be anticancer mechanisms. Off the top of my head senescence, apoptosis, and telomeres. The fact we don't all die of cancer earlier is a result of this.

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