Fractions and Hyphens how to understand?

One page tells : Fractions are written out in words. They are hyphenated only when they come right in front of a noun, called a “direct” adjective.

Another page tells : Do Not Use Hyphens with Numbers Used as Quantifiers When the number is a quantifier to a noun, do not use a hyphen. ?

I see when it comes before a noun = it is both quantifier and adjective

examples:

The journey takes one and three-quarter hours.

The journey takes twenty-three and a quarter hours. ( the fraction does not come before the noun to say it is adjective)

On the other hand, consider the following example in the same page :

1. It is a two-hour journey.
2. It is a two-and-a-half-hour journey.
3. It is a twenty-three-and-a-quarter-hour journey.
4. It is a 23.25-hour journey.
5. It is a one-and-three-quarter-hour journey.

I agree with the last five examples because all fractions come before the word noun ( journey) describing what journey we are telling about distinguishing it from other journeys.

Suppose I have the following question:

Rewrite the following sentences, adding hyphens where needed.

The report shows that two thirds of the population are under fifty five years
of age.

Is ( two thirds ) in the above example = noun ? Is it adjective ?
They said when the fraction is a noun don’t hyphenate, but the opposite I see, they hyphenate.. ?
So, which rule to follow ?

Once fraction comes before a noun = no doubt = adjective modifying the noun

But, how the rule is when it comes after the noun or comes in front but is followed by ( of + noun ) ? examples : two-thirds of people of the population etc..