18 Again!

About 18 Again!: 81-year-old millionaire Jack Watson switches bodies with his 18-year-old grandson David, leading the two to experience life on opposite ends of the age spectrum. But when David, trapped in Jack's aged body, ends up in a coma, his grandfather must use his meager knowledge of the youth to prevent his family from pulling the plug.

His mind was 81. His body was 18. When Jack Watson found his fountain of youth, it overflowed with comedy.