Seventeen Again(2000)

Overview:

Cat and Gene have been divorced for several years. They are both visiting their son and their grandchildren, Sydney and Willie. Willie's a genius who develops a formula that makes who uses it young again, it inadvertently spills into some soap that Willie places in his grandparents room. When each of them uses it they finds themselves 17 again. Willie had to find a way to reverse the process but will his grandparents want to be old again?