I think the better way would be to make all the upgrades available on age earlier at double price, but the price halves every age.
So in Dark Age you can research M@A for 2x price.
In Feudal: M@A for 1x, Longsword for 2x price.
In Castle: M@A for 1/2, Longsword for 1x, TH for 2x price.
In Imperial: M@A for 1/4, Longsword for 1/2, TH for 1x, Champion for 1x price.
This would make teching into them easier, while also allowing the army comp to get more powerful later in the age.
There should probably be something similar for the research times as well.