Dumb and Dumber To is to be released later this year, 20 years after the original Dumb & Dumber.
This motivates me to ask what is the longest gap between release date of a movie and its sequel. Here, by sequel, I consider only movies where characters are portrayed by the same actors as in the original.
Some other examples of long gaps that come to mind are
16 years - The Godfather: Part II (1974) and The Godfather: Part III (1990)
30 years - The Odd Couple (1968) and The Odd Couple II (1998)
But what is the longest gap?