The relationship between adipose fatty acid levels and age is examined in 635 Scottish fertile women aged 25–48. Fatty acids levels are highly significantly correlated with age. Factor analysis identifies four factors that account for 79.6% of variance in the data. Three Factors show significant regressions with age and patterns of involvement of specific fatty acids suggest that these Factors represent the activity of fatty acid delta‐desaturase enzymes as follows: Factor 1—delta‐9‐desaturase, Factor 2—delta‐5‐desaturase, and Factor 4—delta‐6‐desaturase. Key changes, apparently reductions in enzyme activity, occur through the 30 and 40‐year‐old age groups. Such changes in enzyme function could account for decline in female fertility and increases in body fat and chronic disease common in early mid‐life. Am. J. Hum. Biol., 2010. © 2009 Wiley‐Liss, Inc.