I just tried a month of NMN - this brand called Vitamin Shower which I don't know but I checked and they seem legit.
Myself and my girlfriend both took it every morning, 500mg, for 30 days. We are both late thirties.
I can't say I noticed any benefit. My energy levels already are pretty good and I didn't notice any change during those thirty days. My girlfriend said the same.
It's kind of expensive so based on this month of testing I currently don't think I'll use it again. Maybe in another decade it will be more useful to me.
Or possibly the brand we used is not good, which is always a suspicion when trying out supplements.