I'm pretty sure it was a Disney movie similar to the Tom Hanks movie Big. The scenes I remember: A kid invents a machine and ages his friend to adulthood. The friend somehow gets a job at his school and has a relationship with a female teacher. At the end both he and the teacher regress in age back to his original age. The kid who invented the machine ages himself to adulthood and becomes their science teacher

At a difficult place in his marriage and career, a middle-aged man gets a shot at a do-over when he's transformed back into his 18-year-old body.

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.