18 Again! (1988)
After an accident, 81-year-old Jack Watson (George Burns) find his birthday wish to be young again comes true, and his mind switches into the body of his 18-year-old grandson (Charlie Schlatter).
Burns did the majority of his best work on stage, radio and TV with his wife Gracie Allen. However, most viewers will remember him as a mischievous old man in films like The Sunshine Boys and Oh, God! (1977).
Either of these would better have shown off the talents of the veteran trouper, who died shortly after his 100th birthday, as he's given little screen time in this non-starting body-swap comedy that crept in at the end of the cycle that began with Big (1988) and Vice Versa.