Hello Joy: You could refer to the people as "born before the "eighties" or before the "nineties". You could refer to the people as "born during the decade of the "eighties" or the decade of the "nineties". I think that you may be looking for a specific convention that will serve for this. There may not be a convention where you expect one. I know how you must be thinking. Our English does have circumstances like this where some convention would be a real convenience, but there is no such convention. So you might just have to say; "born in the "eighties". Sometimes you will see a phrasing like this: "He is a child of the Sixties." Other phrases would be like this: "Born in the latter half of the Twentieth Century." (1951 to 2000) If I can be of further help to you, just ask, as these are puzzling sometimes, I know.

Do you mean century or others? which nation would you like to know? Is Taiwanese years as Chinese? Are you sure?

Your question is a mess. Please try to be more precise. People born after 1980 were born after 1980. That includes everyone born from 1980 to 2014. In your second suggestion, people born in the 80's were born in the years 1980-1989. Your second question/suggestion is incomplete. Are you suggesting "in the 80's" or "after the 80's"? It would be best to write a complete sentence in the secondary part of your question, so that people know clearly what you are suggesting, so that they can give you a clear answer.