Say what makes you comfortable. I do think that a younger person would probably feel the need to provide an explanation more so than an older person. Personally, I am at the age where I feel that my life and my choices are my business. That said, I'll come clean and tell you that I have used a medical excuse before, to a few people who were more than acquaintances and knew I drank, but not close friends. This was a year and a half ago when I quit drinking. Now I would not feel the need to do that. Sobriety, as well as age, has made me more comfortable in my own skin. What you say now may not be what you choose to say a year from now. As long as you don't drink, its all good. ☺