18 Again! (1988)

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

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.