The only reasons for staying in low ages are (1) perpetual motion with chateaux is possible (2) providing goods for your guild (3) you find combat in an age particularly easy and like doing the same thing over and over again. 2 is really only possible if you do 1, and unless you know what you’re doing you won’t pull it off. My advice would be to go up the age when you have the resources to do so, and enjoy the game content.
PS Some solutions to progressing the continent map are to get higher combat rating, get truce tower and earn higher age goods to trade down, or find someone to give you higher age goods (guild or friends). None of those things will allow you to complete a quest that needs decorations from a higher age though so you’d have to just accept that the story was over until you went up an age.
About the only reason you’d ever want to use diamonds to save troops is if you had much higher level troops that you really wanted to save. You should probably research your army more, learn the special abilities and what beats what. If you’re fighting manually or semi-manually then you should be able to fight much further in GE, so keep reading and practicing.