There are mostly unofficial. Most of the original material (source code, development tools) have been lost even by companies who own the rights (example: Cinemaware).
Besides, a lot of games where a level editor has been used also need specific code to make the levels work (Lemmings, Gods ...)
You could edit some levels but it would be limited.
Of course that's not the case when the official editor has been made public on game release.