Find a shop with a young assistant, look indignant when you are asked your age, then lie! Anyone over 50 looks ancient to a twenty-year-old, so they will be used to such a reaction from fifty-somethings, so are unlikely to challenge your claim.
I was asked that question when still in my forties, so although my age was genuine, so was my indignation! However I had failed the "have you taken anything for thrush in the last six months?' question in the first shop, so at least knew what the 'correct" answer to that was when I got to this second shop. 