In simple physiognomy, if you want to tell the difference between a boy and a man - study his brows. "Loose" brows, where one brow deviates from the other (one is higher than the other, they move separately during emotional expression) implies maturity, experience, struggle, having gone through adversity, the ability to compromise and consider moral and emotional complexity. "Tight" brows, where the brow seems like it's still intact and moves up and down as one whole - implies a certain virginity in these areas of life. Expect more youthful, direct, slightly ignorant behavior from these. Developing this trait early (in young men) implies a difficult upbringing, struggle, sometimes a traumatic and emotionally distraught childhood. Not having developed this feature into mid-to-late 20s can imply sheltered living, emotional indifference or selfishness, but most of the times it implies inexperience in life, lack of independent living and no burden of responsibility.