Milestone birthdays are significant ages that often hold special meaning in various cultures. Common milestone birthdays include turning 18, which marks the transition to adulthood, and 21, often celebrated as the legal drinking age in many countries. Other notable milestones include 30, 40, 50, and beyond, each representing a new decade of life. These birthdays are typically celebrated with parties, gatherings, or special events to honor the individual. They can serve as a time for reflection on past achievements and future goals, making them memorable occasions for both the person celebrating and their loved ones.