There is huge responsibilty placed on staff who sell age restricted products, so please don't blame them for asking for ID. A mistake on their part could result in them paying a fine (from their minimum wage salary), a criminal record and loss of their job. If the stores' policy is to ask for ID if you appear to be under 25, then I certainly wouldn't proceed with the sale if you were unable to prove your age - the risks are too great! How is the checkout person to know whether you are part of a trading standards/police/in-store sting operation or an older looking 17 year old?