The technologies are called “Copper”, “Bronze” and “Iron” weapons/armour though so it doesn’t fit in my opinion.
If AoM(2) had something like the Imperial Age from AoE3 so an Age 5 the progression ending at “Gold” or some Mystical Equipment would make more sense, like:
Copper/Stone → Bronze → Iron → Mystical/Gold/Jade/etc.
Maybe it’s supposed to be Brass.
Brass was more common in the Arabic world because they had access to the needed materials while it was less common in Europe. I guess that’s what gives it the oriental look.
Or just very thinly gold plated.
It is relatively easy to get very very thin sheets of gold and place them on top of other things especially other metals.
I still agree that it’s too much gold.
But tbh, in most AoE(+AoM) games the Age 3 units look the best.
Age 2 is often very basic and Age 4 is needlessly fancy.
You also have to consider that people will probably most often fight in Age 3 (or at last that seems the intended design) and many matches won’t reach Age 4.