Age
Paper money is a relatively new form of currency and Federal Reserve notes only go back to 1913. The age difference of Federal Reserve notes will not be as great as U.S. notes or coins, but can add some value.
Rarity
More than age, rarity will determine the value of paper money. Notes from years or money series in high demand and little supply will fetch more money than older notes in abundant supply. Other rarities include misprinted notes or notes with other printing errors.
Condition
Notes are graded on a 1-70 scale, with 70 being the most valuable. Factors that lower the grade of the note include fading, discoloration, folds, tears and wrinkling. Notes with a low grade are only worth the face value, while high quality notes can fetch many times their face value.