Ultralights typically fly below 5,000′ AGL (above ground level), but they can climb higher. The FAA requires ultralights to stay below 18,000′ MSL (above sea level), but a pilot needs supplemental oxygen to breathe at altitudes above ~12,000′ MSL, anyway. Most ultralights don’t have in-flight mixture or propeller pitch controls, so as the air thins with altitude the aircraft’s ability to climb decreases. Ideal performance is typically in the 0-3,000′ AGL range, and that’s where ultralights typically fly. For most, it isn’t worth the extra engine strain to climb to higher altitudes.