The program is called "Experience Corps" (see link below) and the study is in the Journal of Gerontology (December 2009). In the study, seniors are paired with K-3 children for 15 hours per week of tutoring both one-on-one and in small groups. Using neuroimaging, Michelle Carlson (an associate professor) was able to show that seniors' brain function improved after participating in the program. A larger study is in process to learn more about the benefits of programs like Experience Corps on seniors' brains.
Volunteering has Health Benefits for Seniors - Volunteering and Senior Health