Energy balance

Adult humans possess functionally active brown adipose tissue (BAT) capable of thermogenesis, with prevalence inversely correlated with age, BMI, and outdoor temperature, and higher in women than men.

You likely have some active brown fat in your neck and upper chest, even as an adult. It burns calories to keep you warm, and it works best when you are younger, leaner, female, and exposed to cooler temperatures. While you can't necessarily 'train' it like a muscle, keeping your living environment moderately cool and maintaining a healthy weight may help preserve this metabolic asset.