This is false.
Young adulthood would be when you are at your peak physical health which would be around 20 to 40 years old. Middle adulthood is considered to be from around 45 to 65 years old. When you are a young adult you have more energy and stamina and are able to fight off illnesses easier. Once you reach middle adulthood your life is quite different, you will have a lot more responsibilities, your immune system won't be as effective as it used to be, your energy levels will have dropped and you quite possible will have some health issue.