The movie is not set in Prehistory but After the End
This doesn't explain why the opening scenes, in an earlier prehistory long before Dug's time, feature dinosaurs - but as the original dinosaurs didn't live alongside humans, these could have been genetically recreated dinosaurs who survived whatever techno-crash destroyed civilisation. At the end, Dug's tribe will be saved by an even more technologically advanced people
They will declare the Bronze Age over and say that the Iron Age has arrived.- ...sort of. After the game ends, Dug, the tribe, and now Goona return to the valley, and the difference in living isn't really played up, but if you're inclined to read into it, there are some indications that the Queen's part of the world is even more technologically advanced than Nooth's.
The movie takes place in a Age of Empires game
The British Bronze Age civilization died out from arsenic poisoning
As noted under Artistic License – Geology bronze isn't a naturally occurring metal, but you can get something similar by smelting copper contaminated with arsenic. And we saw that they make frying pans out of this "bronze" they're mining.