Starting line usually refers to racing games and often is an official line drawn on the ground, where all racers stand behind just before the race starts. However, the starting line of a race can really be almost anything; the edge of the grass, the line of a concrete sidewalk, a handy stick, or the edge of a building. Or even, “when we pass that mailbox start running". It doesn't really matter, as long as all racers agree ahead of time.