18 Again!

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

18 Again! is a 1988 comedy film starring George Burns and Charlie Schlatter. The plot involves a grandson switching souls with his grandfather by means of an accident. This was one of a series of unrelated films, including Like Father, Like Son and Vice Versa, produced in the late 1980s involving a similar plotline.