Wow! 
I am so watching that the first chance I get.
Along the same lines To the Manor Born got a Christmas special a few years ago; 25 years after the show ended. Nothing to do with Christmas; Richard and Audrey were about to celebrate their Silver Anniversery. The actors who played them, as well as Majory & the Rector showed up (the latter for 1 scene). Brabinger, Mrs Poo., and Bertie appeared in photograph form, their actors all having died.
Clerks and Clerks II were 12 years apart.
Kevin Smith saved up enough money over 12 years to film the second one in color.
Wasn’t Zane Cooper in the movie Maverick(1994) really Bret Maverick, since it is revealed he is Mel Gibson’s Dad?
Something like that anyway?
Well, there wasn’t a very big gap between sequels, but Desmond Llewelyn played ‘Q’ in the James Bond movies for 37 years.
Quentin Tarantino has supposedly done this. While he was filming scenes for Kill Bill, he allegedly filmed some extra scenes with Uma Thurman, Vivica Fox, and Ambrosia Kelley with the idea that he might incorporate them into a possible sequel in approximately fifteen years where Nikki seeks revenge for her mother’s murder.
I remember that! Won’t it be cool if he uses the same actress that played the little girl? What will he call it though? “Kill Beatrix” just doesn’t have the same ring to it…
Richard Lester’s musketeer movies:
The Three Musketeers, 1973, and The Four Musketeers 1974.
The Return of the Musketeers 1989.
16 year gap. Sadly, several of the actors have died, so we probably won’t get to see them do The Man in the Iron Mask.
Ambrosia Kelley is fifteen years old now and she’s still working as an actress so it’s a possibility.
Decline of the American Empire was released in 1986. It’s sequel, the Oscar winning The Barbarian Invasions came out in 2003 with most of the cast reprising their roles.
ETA: That’s 17 years for anyone who doesn’t want to do the math.
[zombie]
And now 56 Up is running on British TV! Unfortunately I haven’t found a way to watch it here in the US yet…
[/zombie]
Outside bittorrenting it, I don’t think it is viewable in the US yet. 
It’s a remake rather than a sequel, but Peter Cullen reprised his role of Optimus Prime in the new Transformers films - 20 years after doing the role in the cartoon/animated movie.
Kurt Russel was Snake in Escape From New York and Escape from LA 15 years apart
Tom Savini planed a biker gang member Blades in 1978’s Dawn of the Dead, 27 yrs later he made a cameo appeance as the character’s zombie in 2005’s Land of the Dead. That’s a 27 year gap (though in the highly vauge timeline of the Living Dead films it’s only 3 yrs in-universe).
The 2000 sequel/remake of Shaft had Richard Roundtree reprise the role of John Shaft, 29 years after the original.