If we must figure out when someone is an “adult” and when she is still a “child,” which is in most life situations an irrelevant question to ask, then adulthood should not be measured simply by the number of years you’ve been on the planet, but rather by how large your ratio of love-to-fear, your propensity to choose right action over self-interest and self-preservation, and the degree to which you can be self-honest and genuine in your interactions with others about what you want and what you feel, which is self-esteem.
The appearance of a human being is not always aligned with reality. It can be so easy for the world to judge a book by it’s cover.
Just because someone is older does not make that person wiser. It is an easy and common projection for an older person to pre-judge and assume he is wiser than a younger person. This is a projection because he knows he is wiser than his own younger self. But his younger self is not necessarily the same as the young person standing in front of him now.
For young people struggling with feeling pre-judged, misunderstood and already-defined, some words of wisdom from Eckhart Tolle: “When others define you, they do not limit you; they limit themselves.”