One enters the prime of his/her life,
Not necessarily at a given year
Or at a particular stage
Of physical existence.
Rather, he/she does so
When he/she realizes fully his/her potential
And strives wholeheartedly to achieve it;
When he/she recognizes honestly his/her limits
And is content to live within them;
And when he/she is inspired
By a sense of purpose in his/her life
And is happy in his/her quest to fulfill it.
Where are you in your life circumstances? Let me be of guidance.