Corrected version:

"There is little room for doubt that, in some cultures, children are often told that if they try hard, they can achieve anything they want in their lives."

Explanation:

The original sentence had a subject-verb agreement error. "Life" should be changed to "lives" to match the plural subject "children."

Improved version:

"In various cultural contexts, children are systematically instilled with the belief that relentless effort can grant them access to virtually any aspiration they harbour in their lives." 

Corrected version:

"Hard work is the key to success for young people; it is true that they can achieve success at the highest level."

Explanation: 

In the original sentence, there was a comma splice issue, and the structure was a bit awkward. I added a semicolon to separate the related but independent clauses and rephrased the second part for clarity. 

Improved version:

"Indubitably, diligence stands as the linchpin of achievement for the younger generation, bestowing upon them the power to ascend to the zenith of success." 

Corrected version:

"Secondly, if young people really try hard to achieve something and this builds confidence in them, they can handle any difficulty or problem they will face in the future. They don't have any fear of failure."

Explanation: 

The original sentence had  several issues. The conjunction "If" should not be capitalized. "Build" should be changed to "builds" to maintain subject-verb agreement. "Struggle hard" was changed to "handle" for better clarity. "Loss" was changed to "failure" for a more appropriate word choice. 

Improved version:

"Furthermore, when young individuals diligently pursue their objectives, cultivating unwavering self-confidence, they not only surmount present challenges but also fortify themselves to confront future adversities, immune to the spectre of failure."  

Corrected version:

"For instance, a boy and a girl at a young age, after completing their studies, start working at very low wages. After gaining some experience, they begin working longer hours with great effort and start building their own reputations. After that, they no longer fear failure."

Explanation: 

The original sentence had numerous issues with grammar and sentence structure. I added "a" before "young age" for correctness. I corrected the verb tense and structure to make the sentence more coherent and concise. Also, "extra" was changed to "longer hours" for clarity, and "make their own reputation" was changed to "building their own reputations" for smoother flow. 

Improved version:

 "Take, for instance, the narrative of a young man and woman who, post-graduation, commence their careers at meagre wages. Through accrued experience, their relentless toil expands, forging a distinguished reputation and extinguishing all apprehensions of failure." 

Corrected version:

"On the other hand, working hard can also be harmful at a young age. Sometimes, they interpret suggestions incorrectly and make decisions that may lead them astray in life, ultimately causing their downfall."

Explanation: 

The original sentence had multiple grammatical errors and awkward phrasing. I corrected the verb form from "appear" to "can be" for clarity. "Appear harmful to the young age" was changed to "can also be harmful at a young age" for better expression. "They take suggestions in a wrong manner" was rephrased for smoother flow and clarity. Lastly, I combined the last two sentences for better coherence.

Improved version:

"Conversely, an excessive allegiance to the tenets of diligence can wield deleterious consequences in one's youth. On occasion, misinterpretation of counsel may steer them onto ill-fated paths, ultimately precipitating their decline."  

Corrected version:

"Furthermore, achieving success in life is good, but pursuing it stubbornly is not advisable as it can harm one's health. During such times, they neglect self-care, which often leads to failure."

Explanation: 

In the original sentence, there were several issues with clarity and sentence structure. I rephrased the sentences to improve clarity and combined them for smoother flow. Additionally, I changed "doing this" to "pursuing it" for better clarity. 

Improved version:

 "Moreover, while the pursuit of success in life is unquestionably commendable, an obstinate chase for it can take a toll on an individual's well-being. In such phases, self-care often takes a back seat, exacerbating the perils of failure."  

Corrected version:

"In conclusion, hard work can have both advantages and disadvantages. To some extent, the benefits of hard work appear to outweigh the drawbacks, and if we heed this advice more wisely, we can attain success rather than failure."

Explanation: 

The original sentence had some grammatical errors and awkward phrasing. I made the language more formal and clear. "Overweigh" was changed to "outweigh" for correctness, and "in a better manner" was changed to "more wisely" for clarity.

Improved version:

"In summation, the practice of ardent dedication can yield a tapestry of both merits and demerits. While the merits of industriousness may, to a certain degree, eclipse the demerits, it is paramount to assimilate this wisdom sagaciously, as it can serve as the conduit to triumph rather than adversity."