Conditions that determine what a twenty dollar bill will be worth in 60 years include the condition and exchange rates. Based on twenty dollar bills of the past, a twenty dollar bill will be worth more money in the future if it is a special edition twenty dollar bill.
Special edition bills are often an unusual color or have gold lain in the fabric or paper. Most current twenty dollar bills are not going to appreciate with a sixty year period. The most valuable form of investment is special edition bills or notes from the 1800s. If the value of a twenty dollar bill is in question, an appraisal can determine the value of the currency.