Every human has body odor. The changes in body odor in aging people has been traced to a fatty acid known as palmitoleic acid. The person in the 30 does not secrete a great amount of this substance, but once a person reaches 40, the volume rises immensely. The body odor increases as a person ages due to palmitoleic acid. Over time, this fatty acid is decomposed by bacteria living on the skin or by lipid peroxides, which are more abundant in older people, making a substance called nonenal that produces a disagreeable greasy and grassy smell, almost like of old books.