It's common in most cultures to recognise, or even set, a time when a child reaches an age to be able to speak on their own behalf.
It's intention to define a time when children can be held accountable to society for their words and actions (instead of their parents being held accountable). One description that's used is "age of accountability".
This falls in the transition from childhood to adulthood, beginning at puberty. At about the age of 12 logic and abstract understandings are dawning. Wisdom is perhaps some way off!
These are the adolescent years of multi-faceted transitional growth.
With self-discovery, independent decision making and responsibility for personal actions builds confidence. Some time during these years the person is judged to have "come of age".
"Of age" thinking is held by societies and philosophies. In New Zealand some "of age" things, such as marriage and qualifying for a driving licence are laid down legally.
This thinking is shown in laws requiring parental approval through to laws giving full independent decision-making and self-responsibility.