There are several synonyms that can be used in place of "attain maturity." Some options include "reach adulthood," "achieve full growth," "develop fully," "mature fully," and "come of age." Each of these phrases conveys the idea of someone or something reaching a state of maturity or completeness. "Reach adulthood" specifically refers to a person coming into full adulthood, while "achieve full growth" and "mature fully" could be used to describe anything from a plant to an idea. "Develop fully" suggests that something has reached its full potential, while "come of age" has a more cultural connotation, often associated with achieving certain milestones or legal privileges.