One-hundred-year-old houses and antique furniture such as China hutches and rolltop desks often had locks that would only function by using a skeleton key. If you're looking to buy or sell items with skeleton key locks, it is good to know how old they are so you can match the correct key to the locks. A bit of research will reveal the age of the keys, and you'll be able to relay this information to whomever asks.
Determine the age of the house whose locks you are curious about. Ask the real estate agent who last sold the house, or find information from the county clerk about when the house was built to get an idea about when the locks were put in. Real estate agents may have more up-to-date information, such as if the locks or doors were replaced over the years. If you find out the year the doors and their hardware were installed, you'll find out the age of the skeleton keys that match them.
Find the date of the furniture in question. If you're trying to find out the age of a skeleton key that matches your great-grandmother's china hutch, or the rolltop desk your grandfather gave you, ask your relatives if they know when the pieces were purchased. If the furniture was purchased new by your family members, someone might remember when and where, and you'll be able to deduce the date of the skeleton key from that information.
Take the locks or keys to a locksmith. If you are able to take a lock off a door -- be careful not to break the mechanism -- or you can bring the piece of furniture with you, a locksmith who has studied the different types of locks made throughout the years should be able to tell you when a lock was made and how old the key for the lock would be.