What makes a stamp valuable?
The normal market forces of supply and demand. These are all functions of scarcity, interest, condition, fashion etc etc. understanding that unlike many gold and silver coins, stamps have no "intrinsic" value.
Although you haven't
inherited these stamps, you might, nevertheless, want to take a look at
http://www.inheritedstampcollection.com/index.htmlIts a quick read and what you will see is that it is the rare stamp, issued after 1940 or so, that has much value and unless you've put value into a collection, it is unlikely that you'll find much value today, despite the passage of all these years. The one thing that does not drive value is the actual age of the stamps.
If it turns out that there isn't much value here, you still have options to explore which the link will highlight for you.