It is a common belief that a company can benefit from young workers. However, there is a more persuasive argument that elderly members are better because they are experienced people.

>>>CORRECT~!^^

On the one hand, youthful staff members are beneficial for a company.

>>>REVISED:  On the one hand, young employees can offer several advantages.

(More natural phrasing; "youthful" sounds unnatural in this context.)

 This is because they are used to technologic devices and have open mind to advanced technologies. Therefore, they can deal with any technological challenges that happen in their company and easily adapt to them.

>>>CORRECT~!^^

 For example, with development of social networking service (SNS), advertising on the Internet has become popular. 

REVISED:  For example, with the rise of social networking services (SNS), advertising on the Internet has become popular. 

Some startup companies, most of employees are inexperienced but have SNS operating skills, advertise their company or products on SNS, whereas major companies still focus on TV commercial advertisement. As a result, startup companies’ ads are more effective due to influence of SNS.

>>>CORRECT~!^^

 Nevertheless, I support the idea that aged workers are more helpful. 

>>>CORRECT~!^^

The main reason is that they have more knowledge and practice than younger people. 

REVISED: The main reason is that they have greater knowledge and practical experience than younger people. 

more knowledge and practice" - "greater knowledge and practical experience"
(More formal and appropriate.)

Furthermore, when a problem happens, an effective way to solve the problem is listening to their know-how. It could be helpful to find the way. For instance, according to a poll, the majority of respondents ranked elderly coworkers than young members when asked about which member do you prefer to get help in the event of problem. 

REVISED: Furthermore, when problems arise, an effective way to resolve them is by relying on the know-how of older employees. Their experience can be especially helpful in identifying practical solutions. For instance, according to a poll, the majority of respondents preferred older colleagues over younger ones when asked whom they would turn to for help in the event of a workplace problem.

*when a problem happens" - "when problems arise"

*an effective way to solve the problem is listening to their know-how" -  an effective way to resolve them is by relying on the know-how of older employees"

REVISED: In conclusion, it is undeniable that senior staff play ca crucial role in their companies.

• undoubtable" - "undeniable"
  (More natural and commonly used in formal English.)

• "aged members" -"older employees / senior staff"
  ("Aged" sounds outdated; "senior staff" is respectful and professional.)

• "have important parts" - "play a crucial role"
  (More idiomatic.)

 To sum up, while it is apparent that young employees are useful to their team, it is undeniable that elderly workers play a vital role in the company.

REVISED:  To sum up, while it is apparent that young employees bring valuable skills to the workplace, it is undeniable that senior employees play a vital role in the company.

"are useful to their team" → "bring valuable skills to the workplace"

(More formal.) 

"elderly workers" → "older workers / senior employees"( avoid "elderly" in professional contexts.)