Where does “青銅器時代” come from?

青銅器時代 (Japanese) comes from Japanese 青銅器, from Japanese 器, from Middle Chinese 器.

Ancestry of “青銅器時代”, step by step

Step	Language	Word	Meaning
1	Japanese	青銅器	bronzeware
2	Japanese	器	device; instrument; container, vessel; a tool, a...
3	Middle Chinese	器	—