There are many people who prefer having successful careers rather thanto having children.

Typically the construction is "prefer X to Y."

If they had only one option to select, they would choose a successful career rather than havingover children.

Or "choose to have a successful career rather than to have children." This sounds better because there is a stronger sense of parallelism.

This is because when they have a job, they can earn money, while having children means that they have little time to concentrate on their careers and are unable to earn as much.

As indicated by the other set of corrections, the logic of the original statement wasn't very clear.

This is a reason why many Japanese women marry in their forties.

There are mMany people who prefer having successful careers rather thanover having children.

The grammar of your sentence was fine, I think editing it this way is more streamlined. "There are many people who" is a long way that says the same thing as "Many people".

If they hacould only select one option to select, they would choose a successful career rather than having children.

The first phrase of the sentence could me more clear; the way it is now could be interpreted as "one option to select (from)".

"Rather than having children" is redundant because you already established this dichotomy in the last sentence.

Overall I think this sentence is probably not necessary since it says basically the same thing as the last one.

This is because when they have a jobfocus on their career, they can earn money while hre money. Having children means that they have little time to concentrate on their careersmust sacrifice income.

In your original sentence it starts with "when they have a job" which implies the trade off if they decide to have kids would be no job. However, you end the sentence with "they have little time to concentrate on their careers." This phrase makes the tradeoff seem to be just about the amount of money they make, rather than if they can have a job or not. The way I corrected gets rid of this difference and makes it clear the tradeoff is about the amount of money, not the possibility of the job. My correction is quite nitpicky, but I think it makes your writing stronger.

This is a reason why many Japanese women marry input off marriage until their forties.

Based on the last sentence of your entry, I think this way is better. You later say these women have difficulty finding a partner, so saying here that they "marry" does not imply these same struggles.

Alternatively I might write: "This a reason why many Japanese women do not try to get married until their forties."