Is early adulthood and adolescence the same thing?

No, adulthood and adolescence are two different stages of growth.

Adulthood is the stage when a body is fully grown and its the peak of growth, that is the end. After that comes old age. So adulthood is different.

Adolescence is the stage where the real growth of the body takes place. It is the stage of growth where the body reaches maturity and then it accommodates itself to the changes and gradually goes forward towards adulthood.

Early adulthood is the age of 17 to 20 or more. Adolescence is the stage of age of 11 to 16. In this period, the body takes its first step to maturity.