The word antique is not only a judgment of age, but also historical importance. For example if a 100 year old table comes from a common family's household, it would probably not be worth very much money. However if the same table came from a noble or royal household, it could be worth a lot of money and be considered antique. Aesthetics are also taken into account when judging antiqueness.
It also depends on what country you are in. An American antique and a European antique could be considered very different. Because of Americans much shorter history an item would take less time to become an antique whereas in Europe it would take longer. Popular antiques in America for example are items from Civil War whereas in Europe Greek or Roman antiques would be considered very desirable.
The materials used on a specific item are also a huge factor in considering whether something is antique. If a piece of furniture is made of mahogany, the mahogany furniture is likely to be more antique if the item is made out of aluminum for example. This is because, if properly treated, the mahogany furniture will last a lot longer than the aluminum alternative and therefore is more desirable.
There are several definitions of antique, and they are all different to certain people. A family heirloom for example would be a much more valuable antique to that family and less valuable to another. But a general rule of thumb is that an antique must have historical value whether it is a part of a royal household or has survived a common family's house fire from generations ago.
As far as time goes, as a rule of thumb, if an item is over 100 years old, you could well have an antique or some sort. However, old items under 100 years old are considered collectibles. Again though, there are certain exceptions such as certain items from World War II which could easily be considered antique.
The most important thing to take into consideration if you think you may have unearthed an antique is your own actions when looking after it. Simple things such as cleaning the item could massively devalue the potential antique. Also, sometimes repairing a broken antique can take away some of its value. A broken antique tells a story whereas a repaired one will not have such a justifiable story.
Finally, make sure your antique is not a reproduction, reproduction furniture and jewellery is a very large market and chances are the item you have may not be original but instead brand new, or at best old but mass produced.