A simple guideline for beginners about the possessive case and ownership of things or a thing belonging to a person:
John has a car. = John's car is red. Not Jim's.
Please note: In speech, there is no need to REPEAT the object.
John has apples and oranges. John's apples are on the table. Jim's aren't. They're in the garage.
Possessive case and objects or things (this is the area where learners run into trouble):
The door handle is broken.
We tend not to say: the handle of the door. That would only be if there is some doubt about it or ambiguity in a particular context. "The handle of the front door is what I'm talking about, not the back door".
When a word is closely associated with the thing, we tend to use two nouns: door handle, car door. This happens a lot. For example: the results of the lab tests becomes: the lab test results. An adjectival use is created from the noun: lab tests.
However, it can get tricky:
What are the parts of an engine or car?
What are car or engine parts?
Sometimes, the meaning will change. Car parts or engine parts are things that can be replaced. BUT parts of a car can also be understood as: the engine, the hood, the roof, the chassis, and not something replaceable as in something you can buy at an auto parts' store. The parts of a car are not necessarily car parts! This can only be learned through experience. There are no hard-and-fast rules for it. In fact, auto parts' store is also written auto parts store without an apostrophe but you need the s.
(The only good news about the word parts and part, is that it is one of the few causing major issuers for ELLers.)
The hospital's x-ray machines are very new.
Here, the normal possessive case is used. That is because x-ray machines are not "part" (there's that word again) of a hospital in the same way that a handle is part of, or associated with, a door.
The ship's engines have been replaced.
**The opinions of editorial writers"= Editorial writers' opinions.
In the plural, there is no s added after the apostrophe, generally speaking (there can be but it is too advanced for a beginner's guide).
Iron Age=a proper noun, is already accepted. It never was the Age of Iron as a term. Of course, you could use The Age of Iron in a title, if you chose to, but Iron Age is in the canon. Therefore, there is no doubt about it. Just like: The Gilded Age. These are historical proper noun terms. **In deciding on terms like this coming from Spanish, you have to check the term. La Edad de Oro would be: The Golden Age, these are specific terms used in historiography/mythology, etc.
In English, they never had OF in them. La edad de oro del capitalismo, for example, becomes the golden age of capitalism. These kinds of term require being checked for usage by non-English speakers.
Literary terms and titles are another matter, slightly, and call for separate consideration.