Both the Aztec and Mayans used metals, I don't believe they could be considered stone age although I agree with the premise of your argument.
Not for tools. The MesoAmerican Indians worked with silver, gold and occasionally natural copper because it was maleable -- but only for adornment and ornamentation. Tools and weapons were made of wood, stone or obsidian, or occasional odd materials such as sharks' teeth or sawfish snouts.