After you answer the question with an honest yes or no then you can seize the opportunity for a meaningful discussion with you child. This is the time for you to explain how you decided whether or not to drink when you were a teen. Our children are curious about whether we drank, but what they really want to know is what is was like for us when we were kids. They want to know that we struggled with our decisions just like they are struggling. And believe me, they only ask the question because they are struggling with their own decision.
If you did drink, be sure to explain the negative consequences that occurred when you drank as a teen. You don't have to go into graphic details, but be sure to make the point that it was the wrong decision to drink as a teen. And don't just focus on the fact that you were grounded for a month and that you had to "do your time." Discuss the hangovers, or the arguments with family and friends, or the physical fights, or the vomiting, etc. Admit that you made a mistake.
If you chose not to drink, explain that it was a difficult decision. Explain that you had friends who did drink and comment on the fact that it was hard to not go along with the crowd. Also explain that you were able to maintain friendships even though you didn't drink alcohol with your friends. Be sure to mention that you were proud of your decision. Let them know about the negative consequences faced by your friends who did drink that you were able to avoid. You can also point out to your that making a decision that is good for them is not always easy in the moment, but it is something they can be proud of for themselves.
Finish up the conversation by saying directly, "I know you'll have some difficult decisions to make while you're a teenager and even when you're an adult. I can't make the decisions for you, but I will always talk to you about them and I am confident that you will make decisions that are the best for you."