Baking powder does not last forever. Because it’s sensitive to moisture and humidity, it generally has a shelf life of between six months to one year. Baking powder should be kept in a cool, dry place, such as inside a cabinet, and should be discarded when it is no longer active. (Its cousin, baking soda*, has an indefinite shelf life, although some manufacturers recommend changing it every three years.)
To test if baking powder is still active, spoon 1/2 teaspoon in a bowl and pour 1/4 cup (60 ml) of boiling water over it. Right away it should bubble up violently. If it does, it’s still good. If it doesn’t, discard it and open a new tin.