Your body odor is a complex mixture of bacteria and secretions from your skin glands, and these change with age, according to a 2012 article in PlosONE. For example, your sebaceous gland has oils and fatty acids that become more active during adolescence and peak when you’re an adult. These glands are less active when you reach your 70s.
A 2001 article in the Journal of Investigative Dermatology studied people aged 26 to 75 and found a specific compound, 2-nonenal, only in people over the age of 40. This nonenal is what causes the musty smell. As people get older, there’s an increase in omega-7 fatty acids and lipids that oxidize on the skin’s surface. The study suggested that the nonenal substance is produced by the breakdown of these fatty acids.
Researchers believe hormone imbalances produce more of these fatty acids on your skin. As you age, your body also doesn’t produce enough antioxidants, so you’ll have more of these fatty acids breaking down on your skin (per Franklin Park Retirement Communities).