Coins that are hundreds of years old can be worth a bit of money, but generally because they are rare.
Say, I had some notes of francs or Zimbabwe dollars or some other currencies that are no longer used.
Will they gain value in the future? Or will the fact that today we have much better preservation methods and techniques mean that there will always be plenty of these currencies available for them to never be rare enough to be valuable?
The value of Coins & Currency is more of Demand Vs Supply Situation. The value of Coins is not just due to it being old, but also
For example the 1876 US Nickel around 300,000 were minted. The 1916 US Nickel around 400,000 were minted. However the 1916 Nickel is at more premium than the 1876. This is because there are many collectors who want to complete the collection post 1900. They can never complete the 1800 to 1900 collection as other year coins are not available.
It is hard to tell which currency will catch craze amongst collectors. That's what will drive the prize. For example if 50 years from now, Zimbabwe becomes a leading developed country, there will be craze / nostalgia to collect things of past.
Certain coins like the American Double Eagle are a craze because the design is very good, its made of Gold, has history to it. It's sold for USD 7-8 Million, Or the USD 1 Dollar uncut sheets [16 notes in a sheet], the sheet goes for around 1-2 Million USD
As an individual if you have some coins / currencies keep them more as a hobby than as something that would return value.
Investments in Collectibles and Art is an emerging form of investment and needs to be looked at separately and shouldn't be mixed with a simple hobby.
Coins have the minimum value of the metal they are made from.
Bank notes (paper money) would only be valuable when it becomes rare. And there isn't a good way to predict how quickly something like Zimbabwe dollars will become rare (that I know of at least).
