Do counterfeit pens work on old money?

Counterfeit detection pens typically work by reacting to the starch in the paper used for modern currency, which contains specific security features. Older money, especially notes issued before certain changes in paper composition, may not trigger the same reaction. Therefore, while counterfeit pens can provide some indication, they are not reliable for detecting counterfeit old money. For accurate assessment, it's best to use more specialized methods or tools designed for older currency.