We present the results of examining body composition variations in a large self-selected sample of 24,128 UK adults aged 20-70, measured by Bio-impedance equipment. The expected pattern of increasing body fat with age in both men and women is found, with a fairly steady increase across all age decades. We also show that although lean mass percentage inevitably declines as fat mass percentage increases, the pattern for absolute lean mass also shows a decrease with age, accelerating after about 45-50. Little regional variation is found, although the variations are significant in women. Examination of the association between fat percentage and body mass index shows that at a fixed BMI, body fat increases with age. This illustrates one of the weaknesses in using BMI as an indicator of body fat.