Actually, in almost all tissues, including the brain, folates are required for one-carbon transfer reactions, which are essential for the synthesis of DNA and RNA nucleotides, the metabolism of amino acids and the occurrence of methylation reactions.

Studies show that the prevalence of folate deficiency is high among individuals aged ≥65 years mainly due to reduced dietary intake and intestinal malabsorption. Population-based studies have demonstrated that a low folate status is associated with mild cognitive impairment, dementia (particularly Alzheimer’s disease) and depression in healthy and neuropsychiatric diseased older individuals. The proposed mechanisms underlying that association include hyperhomocysteinemia, lower methylation reactions and tetrahydrobiopterin levels, and excessive misincorporation of uracil into DNA.

More details pls click https://pubmed.ncbi.nlm.nih.gov/25939915/