"It takes generations."
Translation:Det tar generationer.
I think I could solve it myself: "Det" is not only used to refer to an "ett"-word, but also to refer to an action. Here is an example that expresses that quite nicely: Jag köpte en ny cycel. Det är mycket bra. Jag köpte en ny cykel. Den är mycket bra. Both ways are possible. First one says, it is good, that I bought a new bike. Second one says, that the new bike I bought is very nice. So first one actually refers to the action of buying the bike, not the bike itself.
Taken out of context like this, it's a little confusing... but I think it's because "det" isn't actually referring to "generationer" in this sentence, so it doesn't have to agree with it. I'd assume that it's actually referring to another unseen subject or concept. Though I'm only learning myself, so I could be wrong!
You are correct, we start the sentence with "Det" when referring to something general, undefined, in contrast to when refering to a specific 'den'-word, 'Denna förändring' - den tar generationer.
A question for a native: What period of time are we speaking of when it was "jag tager" instead of "jag tar"? I remember seing an old Swedish movie years ago, with a wedding ceremony and the broom said to his bride something like "Jag X tager dig Y..." He even pronounced it like "jaG taGER diG".