What Is Baby Fat Patient’s parents always asked me at what age does baby fat turn into regular fat? Baby fats real name is brown fat it makes up about 5% of the body mass of an infant. It's mother’s nature way of keeping baby warm because it burns calories per heat. It also give baby that pudgy look that so darling cute. One pound of stored body fat equals 3500 calories. That’s a powerful source of energy as the baby starts to crawl around and grow. As an estimate baby fat should be completely gone by age three after that keep an eye on the height and weight growth charts put out by the CDC to make sure they're not following in the obese ranges.