We don’t use the word much before an adjective except ...

[Odessa Dawn]

As a general rule, we don't use "much" before an adjective.

How angry were you?
I was very angry.

How surprised was your brother?
He was extremely surprised.

How happy are you?
I'm quite happy.
https://www.usingenglish.com/forum/ask-teacher/184901-how-suprised.html

Then we have ...

Re: older/elder Older is the standard comparative of old.

My brother is older than I am.
My car is much older than yours.

Elder (adjective) is normally used only of members of one's close family, especially siblings. It is never used with than.

My elder brother is a teacher.
I have two brothers. The elder is a teacher, the younger a miner.
https://www.usingenglish.com/forum/ask-teacher/136910-older-elder.html
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Definition of older | Collins English Dictionary

I couldn’t grasp the exception here. Will you help me by setting me a rule when the word much comes before an adjective, please?

To be honest with you, I want to make emsr's statement a universal rule but in 5jj's post the word much precedes the adjective older. Does the below statement make sense?


We don’t use the word much before an adjective except in comparison for emphasis.

Example:
Your book is much bigger than mine.
Definition of big | Collins American English Dictionary
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[probus]

As you noted, much is used with the comparative form of adjectives. You can understand and remember this usage by thinking of it as by much, i.e. as by a significant margin. In other words, much colder means colder by a great deal.

This usage also applies to the superlative of adjectives, and it is done by adding the, in order to make it singular. Examples:

much the easiest means by far the easiest
much the strongest
much the fastest etc.

Much is also used with what I think of as past participles of verbs, used as adjectives. Apparently a lot of people now call these passive forms of verbs, but I prefer to think of them as past participles. This probably just means that I am old or a simpleton or both. Examples:

the much abused word ...
Picasso's much admired paintings ...
much given ...

That last one is an idiom and has an illustrious pedigree. For example, King Edward VII was much given to gluttony.

 

[probus]

I've thought of some exceptions to my last reply.

Worried, excited and tired all take very rather than much almost always, and there are probably others of this type. I think they all started out in life as past participles, but subsequently managed to acquire the status of simple adjectives in their own right. Some dictionaries don't seem to agree but that's the only explanation I can offer for these exceptions.

Then there is another group of participial adjectives that may perhaps be in transition. You can be much concerned or very concerned with little or no difference in meaning. You can't be much delighted without sounding antiquated and unnatural, and I am uncomfortable with very delighted. But you can be extremely delighted, which I think means that delighted is migrating toward the very camp.

To sum up, there must be a lot of exceptions to be learned for any general rule that could be proposed.