what is the meaning of the punctuation? please
Let us get down to fundmentals and agree that the young are after all human beings--people just like their elders. there is only one difference between an old man and a young one:...

Does the people mean the young?
what is the meaning of the --?
i knew the dash, but the dash i saw are often put in the head and tail of one sentence, and it is often short line to present, just like the following sentence
People are always talking about 'the problem of youth'. If there is one - which I take leave to doubt - then it is older people who create it, not the young themselves.
why does the last words use the one? dose it stand for some mood or emotion? why is it "between an old man and a young man"?

Question

what is the meaning of the punctuation? please
Let us get down to fundmentals and agree that the young are after all human beings--people just like their elders. there is only one difference between an old man and a young one:...

Does the people mean the young?
what is the meaning of the --?
i knew the dash, but the dash i saw are often put in the head and tail of one sentence, and it is often short line to present,  just like the following sentence
People are always talking about 'the problem of youth'. If there is one - which I take leave to doubt - then it is older people who create it, not the young themselves.
why does the last words use the one? dose it stand for some mood or emotion? why is it "between an old man and a young man"?

Answer

Hello! A few answers to your questions:
1. Yes, the "people" mentioned in the second line refer to "young" people. 
2. The "--" is sometimes used in place of a colon (:). In the above paragraph, it seems to be used that way. ( It can also create a pause within a sentence, but it is mainly used that way in poetry.)
3. That is an interesting opinion! You can definitely make an argument to support your opinion.
4. In this case, "young one" is just an expression. "One" can sometimes stand in a sentence for "he," "she," or "a person."
For example, I could say, "When he gets mad, he yells." I could also say, "When one gets mad, one yells." They both mean the same thing. In the sentence above, "one" and "man" would both be correct.