The Sturmey Archer hub will have the month and 2 digit year of manufacture on it. The bike age will generally be less than a year after that date. Picture below shows a hub manufactured in April of 1953. If you cannot easily make out the date DO NOT use anything abrasive or acidic to clean the hub in order to reveal it. Protect the tires and other parts and use mineral spirits or similar if the hub is grimy, and something like baking soda paste if it's slightly rusted.