There was a scene in Twin Peaks where Cooper has grey hair and a subtitle reads "25 years later." Why 25?

Why not?

Here may be some reasons: The number of 25 has a lot of symbolism…

Number 25 Symbolism, 25 Meaning and Numerology

Or maybe it is a because it is a perfectly cromulent number:

25 (number)

Whatever the reason, it is what it is – and I’d wager it just “worked” when they were writing that scene.

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