what make nitrous oxide decrease as we get older, what are the effect, what are the solutions

The short version: the question asks about age-related decline in the body's own nitric oxide (NO), not about the anesthetic gas nitrous oxide (N₂O), and the evidence provided covers only nitrous oxide, not endogenous NO.

The evidence you received touches on endogenous NO [1] but only in a tangential way—as a mechanism for how nitrous oxide increases cerebral blood flow [1] and in the context of septic shock and vascular biology. It does not contain a single study that measures how the body's own NO production changes with age, what causes it to decline, or what treatments restore it. That is a major gap. Endothelial nitric oxide synthase (eNOS) activity is known outside this retrieval to drop with age due to oxidative stress, inflammation, and loss of the glycocalyx—all factors that degrade the zeta potential and colloidal stability of blood. But these records do not give rates, mechanisms, or trial results for the age-related NO decline itself.

Because the retrieval does not match the question, the honest answer is that the evidence here does not answer "why nitrous (meaning endogenous nitric) oxide decreases with age." If you were asking about nitrous oxide (the anesthetic gas), these records show it is a very different molecule with very different pharmacology. The bottom line: the literature outside this retrieval does document age-related NO decline, but these studies are not it. Without a study that actually measures eNOS or its substrate L-arginine in older versus younger humans, no confident call can be made from what is here.

My call: the available records do not address the question. Confidence: not clear.