In Texas, as in many other states, it is illegal for you to drink and drive if you are under the age of 21. The state uses your blood alcohol concentration level to determine if you are driving while intoxicated. If there is any amount of alcohol in your system that can be detected, you could be charged with DWI. You will also run afoul of the law if you are found to have an open can of beer or bottle of alcohol on the passenger seat of your car. In short, alcohol and driving do not mix, and if you think otherwise, you could be in for some serious legal difficulties.