You have no chance.

I went to Vegas a few times w/ parents while still a teenager and was kicked out of numerous casinos. I think the longest I was able to play a slot machine was about 30 seconds before I was spotted. They are very serious about this.

Luckily there is a lot to do in Vegas now for those under 21. If you really want to gamble, some of the arcades have those machines where you drop a quarter in and and try to make it knock others coins off the shelf.