If you are required to have a license or be 18 I would take a 18 year old with me or an adult but every walmart is different for I see no reason why they should Id you anyway or not let you have a duplicate key.
I have been going to Walmart my whole life and they never refused to make me a key because of age, and they never asked me for an I.D. . No joke. It's just a key. There is no reason for them to card you!