Garth Brooks' daughter Allie just had the weekend of her life. On July 28th, she celebrated her 21st birthday by joining her dad's background singers at his show in Los Angeles. After Garth introduced her to the crowd, he and the audience serenaded her with "Happy Birthday." But her exciting night didn't stop there. Shortly after exiting the stage, Allie's boyfriend Jonathan got down on one knee backstage and asked for her hand. Luckily, the adorable moment was caught on camera.