Find out how long they are willing to commit themselves. Point them at immediate results and benefits. Don't make the activity seem too radical or the work sound too technical. Demonstrate good volunteer-management practices. Be prepared to disabuse the volunteers of notions that the environment is a young person's concern, that most environmentalists are radicals, that older volunteers are ineffective, and that with no environmental training, a person can contribute little.
Age is no barrier to performing community service.
Seniors can give to a community as well as take. Many seniors have an even stronger commitment to future generations than do younger people. If so, it will probably want to tap into existing environmental groups. If so, it may want to create its own, free-standing organization. Thus, the mission can dictate the type or organization.