Tags can't be created before they are needed. They only exist because they have been added to questions.
Therefore, if a tag is needed it should be just added to the question and it will exist to be used by everyone else.
However, (and it's a big however) these "decade" tags smell like meta tags which are a "bad thing". I'm not 100% sure they are meta tags, but they are very close to being such. The test is
Does the tag work when it's the only tag applied to a question?
If the answer is "no" then it's a meta tag and shouldn't be applied.
Having said all that, perhaps if the tag included the age as well - e.g. 1920s-electrics (though I'm not sure that's actually a good tag either, but you get the idea) - then that might be the way forward.