The Merriam-Webster dictionary defines middle age as the period of life between 40 and 60 years of age. Other organizations have different criteria, such as the American Psychiatric Association that considers 45 to 65 middle age. The U.S. Census classifies people between 35 and 54 as being middle age.
Middle age is the life period between young adulthood and old age.
Though the exact ages that middle age is said to occur vary, the body undergoes noticeable signs of aging between 40 and 65 years of age. Skin elasticity decreases, muscle mass decreases, body fat increases, fertility declines, strength and flexibility decrease and hair starts to turn gray. Menopause occurs in most women when they reach their mid-40s or early 50s. Children born to middle-aged parents have an increased risk of certain disorders. The risk of miscarriage is also higher.