Big Brothers Big Sisters of San Diego County Mission: Provide children facing adversity with strong and enduring, professionally supported one-to-one relationships that change their lives for the better. Big Brothers Big Sisters is the oldest, largest and most effective youth mentoring organization in the United States. We have been the leader in one-to-one youth service for more than a century, developing positive relationships that have a direct and lasting impact on the lives of young people. Big Brothers Big Sisters of San Diego County serves children, ages 7 through 18, in six distinct programs: Amachi, Bigs in Schools, Community Mentoring, High School Bigs, Operation Bigs and Sports Bigs.