With ageing, a slight decrease in cognitive impairment is seen due to increase in level of:
Homocysteine and Dementia

Homocysteine is an amino acid that has been shown to have direct and indirect neurotoxic effects in vitro at high concentrations.
Several observational studies have consistently shown cross-sectional association between high levels of serum homocysteine levels and impaired cognitive function (dementia). This association has generally been greater among elderly than among the middle aged.
Current data demonstrate that homocysteine is strongly associated with cognitive dysfunction in ageing. However, it is not yet clear whether this association is causal or whether lowering homocysteine levels improve cognition.
Vitamin B12, folate and homocysteine are closely interlinked and supplementation of vitamin B12 and folate in the diet has been used to reduce homocysteine levels.
Methionine
Cysteine
Homocysteine
Taurine