My youngest brother is currently exposing my nieces to classic TOS. I hear it's going well, although the Salt Vampire spooked them. An older niece got hooked on Smallville,
even though she didn't know anything about comic books or Superman.
Hard to say what might appeal to them when they're older. The Twilight Zone
is timeless, of course, provided they can get past the black-and-white business. And I like to think that Buffy
will age well. Witty dialogue, engaging characters, and teenage angst never go out of style.
Nostalgia aside, some stuff ages better than others. Most of the Trek shows hold up well, as well as classics like The Avengers
or The Outer Limits,
but I wouldn't dream of subjecting a younger person to original-recipe Dark Shadows
or The Green Hornet,
no matter how I might have enjoyed them back when I was a kid.
Not sure about Kolchak:The Night Stalker.
The dialogue and writing probably hold up, but the bargain-basement monsters and makeup might be a turnoff for the younger generation.
P.S. I don't think it's the length
of older shows that's an issue as much as the pacing. Older shows and movies just tend to be slower and talkier by modern standards.