Who came up with these ideas?
Did the Romans call themselves ancients? Would a person living in medieval ages have known it was the medieval ages? We're these ideas in existence during those eras or were they created afterwards?
"Ancient" is an entirely relative term with no real borders. It just means "particularly old," so no one has ever called themselves ancient. Ancient Egypt includes the first pyramid in ~2670 BC, ancient Peru includes the Chavin de Huantar temple in 1000 BC, ancient Greece include the Parthenon in 447 BC, and ancient Rome includes the Pantheon in 127 AD. There's conventions about when you wouldn't use "ancient," e.g. after the end of a certain Chinese dynasty or the fall of the Western Roman Empire. But no one will fault you if you call Odoacer an ancient Roman ruler or if you say he's not. It's a term with any particular importance.