18 Again!

18 Again! (1988) is a fantasy, comedy directed by Paul Flaherty rated 5.6/10.

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.