Every time I go to my office, I have to take the elevator because my office is on the fifteenth floor, and. I notice that people around me in the elevator are all younger than me.

They look like they are in their twenties or thirties, ands you can rarely see people in their forties.

This makes me very moved.

I don't understand why this would make you very moved. can you please explain this further?

Of course, they are all working in somefor private enterpriscompanies.

If you are working in a government office building, you will not see this kind of situation.

What situation are you referring to? The fact that they work for private companies or the fact they are in their 30's and 40's?

I think if I worked infor a private enterprisecompany, I would probably already get fired at my age, bbe fired already. But if I am still at working at my age, the only possibilityle explanation is that I must be a big boss on my own.

I'm not sure what you mean to say by "a big boss on my own."

We will never ever know why there are significant differences between the two different fields, but I always believe that the young are hope, the young will do things better than the old, so the old should give way to the young.

Again, I'm not making the connection regarding what you are trying to say.

Like in the United States, twoboth of the presidential competitorandidates are all in their seventies. In fact, one is almost in his eighties.

Can't Americathe US find a younger person to be president?

I think it is always better to use the "US" versus "America". There are other people in the Americas, such as Latin America and Canada, therefore, using America is truly egoistic and ethnocentric.

So I bet the United StatesS will go down in all aspects sooner or later.

I'm not sure what you are trying to say. You mean the US will lose its superpower status?

Every time I go to my office, I have to take the elevator because my officeit is on the fifteenth floor, and. The people around me in the elevator are allways younger than me.

Usually native English speakers will try to reduce repetition of words and swap them for pronouns when possible.
I also split the sentence because there were two separate ideas (going to the office vs people in the elevator).
Lastly, changed it to always because it seems like you're talking about a general trend and not a specific incident.

They look like they are in their twenties or thirties, and you can. You rarely see people in their forties.

If you are working in a government office building, you will not seeencounter this kind of situation.

"see" is also fine, but "encounter" is often paired with "situation"

I think if I worked in a private enterprise, at my age I would probably already getbe fired at my age, but. However, if I am still at work at my age, the only possibility iswould be that I must be a big boss on my own.

We will never ever know why there are significant differences between the two different fields, but I always believe that the young are hope,ful, and the young will do things better than the old, so the old should give way to the young.

If you break this rule, you will get bad resultssomething bad will happen to you.

Every time I go to my office, I have to take the elevator becauseas my office is on the fifteenth floor, and people around me in the elevator are all younger than me.

They lookseem to be in their twenties or thirties, and you can rarely see people in their forties.

Of course, they are all working in some private enterprises.

If you are working in a government office building, you will not seecome across this kind of situation.

I think if I worked in a private enterprise, I would probably already get fretired at my age, but if I am still at working at my age, the only possibility is that I must beat's because I am a big boss on my own.

there was no grammatical error I just thought it should be written this way.

We will never ever know why there are significant differences between the two different fields, but I always believe that the youngsters are hope, the youngy will do things better than the older (previous) generation, so the old should give way to the younger generation.

Can't America find a younger person to be the president?

Because it's a post, hence the article 'the'.