According to the poem, the process of growing up involves the attainment of mental maturity and loss of innocence and simplicity. A person is said to be grown up when he has become logical, rational and is able to build his own thoughts. A grown up has the power to distinguish between reality and fantasy.
A grown up individual understands the actions of others just as the poet recognized the hiatus between the preaching and the practice of the adults. He realizes the hypocrisy and the double standards maintained by the adults. A mature individual also asserts his thoughts and opinions.