The division into stone-copper-bronze-iron is a just-so story from before scientific dating, and doesn’t have much bearing, not least because the availability of bronze came and went (probably for economic reasons or the access to raw materials or both), so the bronze age in parts of the world was still going on when Columbus liberated the New World.
That is why the disappearance of bronze should not mark the end of the age. Rather the appearance of a new metal (iron) or rather the ability to obtain it in quantity and the ability to smelt ore into ingots and work iron ingots into tools, should mark the start of the new age.
...so the bronze age in parts of the world was still going on when Columbus liberated the New World.
(((
I guess they want us to believe that all civilizations progressed at exactly the same rates.