Also, almost all hotels require a valid ID that proves you're 21 to check in . So the long and short of it is that if you're under 21, don't go if you're ONLY interested in the gambling scene. First, decide how long you want to be in this neon wonderland. As for a good time to go, well, it's always crowded and it's always hot. It's a desert.
Deserts are hot.
However, because of the searing heat, the summer also tends to be a bit less crowded. Stay away during major holidays, especially New Year's Eve, Memorial Day Weekend, and Labor Day Weekend. It gets downright crazy during those times. Like we even need to ask. If you live on the West Coast or anywhere somewhat close to Vegas, you can save some money if you road trip it.