This should not be flagged as a Space Age Mod; I don't even own Space Age and it works just fine for me. On that subject, why is "Space Age Mod" an independent flag in the mod system? Shouldn't it just be an especially-visible way of advertising the fact that the mod has Space Age, Quality, and/or Elevated Rails as mandatory dependencies?

That wasn't supposed to be this way. I'll look into it over the weekend and either fix it or post an update here.

Okay, this is all turning out to be really strange.
The “Space Age” mod tag appears in the mod portal in the browser and in the game, but when I download and activate the mod, “Space Age” isn't activated along with it. To me, that means I've set the dependencies correctly. But I can't figure out why the tag is showing up in the mod portal.

So I won't be able to fix this right away.
I will continue to look into this and follow up on it.

Oh wow; I was assuming that the mod maker's side of the mod portal had a checkbox that controls whether their mod is flagged as a Space Age Mod. It would be very dumb to do it that way instead of automatically based on dependencies, as I explained in my original post, but you having no control at all over the flag is even weirder! It might be time to ask Wube what the heck is going on.

Error found! The tag is set as soon as any version in the download history includes this dependency.
In this case, it was the very first version (1.0.0).
I've now deleted it, and the tag is gone.