The invention discloses a method for the analysis of N-glycans attached to immunoglobulin G (IgG) or IgG N-glycopeptides from human blood plasma in which relative abundance of two or more glycans is determined, out of total six, and for these glycans it is determined that they strongly correlate with age. The glycans have the following structures: F(6)A2 (GP4): R1, R2, R3, R4=H F(6)A2B (GP6): R1=GlcNAc; R2, R3, R4 H F(6)A2[6]G1 (GP8): R1, R3, R4=H; R2= Gal F(6)A2G2 (GP14): R1=H; R2, R3=Gal; R4=H F(6)A2BG2 (GP15): R1=GlcNAc; R2, R3=Gal; R4=H F(6)A2G2S1 (GP18): R1=H; R2, R3=Gal; R4=NeuAc GlcNAc=N-acetylglucosamine Fuc=fucose Man=mannose NeuAc=N-acetylneuraminic acid Gal=galactose From the results of the analysis, Glycan Age Index (GAI) is calculated, and it is useful for: prediction of biological age of a tested individual; monitoring efficacy of methods that slow down the ageing process; monitoring progression of diseases that are developed as a result of the ageing process advancement, like: inflammatory diseases (including atherosclerosis), autoimmune diseases, tumours, diabetes, arthritis, osteoporosis, and Alzheimer disease; and evaluation of overall condition / health of a body.