Adult/ˈædʌlt/ Definition 1 of 4Pro Tip 1/3Legal Significance The term 'adult' marks the legal age when someone can engage in activities restricted to grown-ups. At 18, you're considered an adult and can vote. Pro Tip 2/3Cultural Variations Age defining adulthood varies across cultures; some countries consider younger individuals as adults. In some places, you're an adult at 16. Pro Tip 3/3Age vs. Maturity Being an adult implies maturity but not all adults behave maturely in every situation. He's an adult, but he acts childish at times.