This is the 1983rd most frequent Portuguese word.
adulto
An "adulto" is an adult; a fully grown or mature person.
Here, 'adulto' is used as a noun to describe a mature individual.
Ele se tornou um adulto responsável.
He became a responsible adult.
In this sentence, 'adulta' is used as an adjective to describe a manner resembling that of a mature person.
Ela age de maneira adulta em situações complicadas.
She acts in an adult manner in complicated situations.
Here, 'adulta' is used as an adjective modifying 'vida' to refer to the phase of life characteristic of adults.