The US Department of Labor has released a detailed report on volunteer work in the United States. This is an excerpt from the report:
By age, 35- to 44-year-olds and 45- to 54-year-olds were the most likely to volunteer (31.8 and 30.6 percent, respectively). Persons in their early twenties were the least likely to volunteer (19.4 percent). Among the major race and ethnicity groups, whites continued to volunteer at a higher rate (28.2 percent) than did blacks (20.3 percent), Asians (20.0 percent), and Hispanics (14.9 percent). The volunteer rate for blacks increased in 2011. For all other major race and ethnicity groups, the volunteer rates were little different from the rates in 2010.
The full report is available here.