The older the stamp is, the more valuable it must be. That is what many non-collectors believe. Even some collectors believe this. However, age has nothing to do with the value of a stamp.
Speaking in general terms, yes, the more valuable stamps tend to be the older stamps. However, that is not always true. There are exceptions. This applies to both US and worldwide issues.
Stamps printed after the 1930s and 1940s were usually printed in large quantities. These issues will never be valuable. They are too common.
Stamps from 1900 to 1940 are a mixed bag. There are plenty of common/inexpensive stamps. However, there are some better items too.
Stamps issued before 1900 are often the most valuable stamps.
Take Scott #231, the 2¢ Columbian from 1893. They printed almost 1.5 billion of these stamps. Yes, that is a “B” for billion. Used copies are extremely common. Even mint copies don’t sell for that much. This stamp is 120 years old and will always be common.
Go further back. Scott #11 is over 165 years old now. A good looking used copy is only going to cost $10 or $20. Even a mint copy won’t require a second mortgage on your house.
Despite their age, these stamps are common. They are present in many collections.
Conversely, Scott #1789B is the perf 12 variety of the John Paul Jones stamp from 1979. Only a few copies are known. When they sell, they sell into the thousands of dollars.
The value of a stamp is determined mostly through demand. I contend that appearance plays a smaller, but important, factor too. When possible, collectors tend to like nice looking stamps. However, age is not a factor in stamp value. You can talk in general terms, but there are plenty of exceptions to the rules.