"What would you advise a young adult (age 18-24) to do to boost her or his credibility when trying to persuade an audience of senior citizens (ages 60+) to volunteer for, and participate in, various community service projects?"

By feeding off their vulnerabilities. Most adults whose ages are 60+ are sometimes lonely. Giving them an avenue to socialize while they volunteer is a very good means of persuading them.

Explanation:

Persuasion is pervasive. We see it in media, gathering, religious meeting, societal functions etc. We have pure Persuasion and borderline Persuasion. The former is done intentional, for instance, consuming an individual to buy a product. The latter may not be intentional to produce an effect. In order to persuade a senior citizen to volunteer and participate in community service, a young adult can feed off their vulnerabilities by taking cognizant of the fact that most senior citizens are lonely and would volunteer and participate in a community service if various methods are put in place for the senior citizens to socialize and meet others of the same age group or an avenue to talk to younger adults /generation. This means of Persuasion would probably convince senior citizens in partaking in volunteering or community service.