Lassiter was born in Turrialba, Costa Rica, where his father Roy, a professional footballer, played for A.D. Carmelita. The Lassiter family moved to the United States after the elder Lassiter signed with the Tampa Bay Mutiny of Major League Soccer. before eventually moving to Temecula, California when Ariel was 13. There, Lassiter attended Great Oak High School, where he played on the school soccer team for 4 years, captaining the team for 2 years. He received multiple accolades during his high school career, including being named two-time Inland Empire High School Player of the Year. Lassiter also spent 5 years playing for Albion SC, a youth soccer club in San Diego, California.
