So I heard once that something is ancient if it's over 250 years old. But that would mean in just over a decade, the United States will be ancient, and that doesn't seem right. The Roman Republic feels ancient, but that's ~2000 years ago. So after many fruitless searches, I come to you, AskHistorians- when's the cut-off for ancient-ness?
It depends on your field and your definitions.
In Classics, 'Ancient History' and 'the Ancient World' often correspond to the period starting say 753BC or 776BC (the putative founding of Rome, and the First Olympiad and start of recorded Greek history in the Archaic period). It then encompasses Mediterranean and Near-Eastern studies through until Late Antiquity. The cut off period varies, again depending on opinion. It's quite typical to consider the fall of Western Rome in 476AD to be the end of 'Ancient History'. It is a convenient 'date' to see Western and Middle Europe transitioning into the Early Middle Ages.
But very few dates in history represent hard and fast 'divisions' like this. There are almost always cross-over periods. Dates just help give you a short rule-of-thumb type categorisation.
Anyway, others here may give you different answers, this is what I use in my field for 'Ancient'.