What is to gain by leveling down an age? I can't think of it unless I am missing it.
One of the main reasons people have requested it over the years is Guild Goods production.
If you want to bring in new players to established Guilds that's going to bring in a cost to
Guild Treasury. New players don't have the same production ability that established cities have. At the same time you don't want to keep buildings from 3+ years ago unless you really have nothing better to place. If you're Aging up you're going to be limited in your options for previous Age goods if you're aiming to keep your city upto date
Even if everyone only down-aged one building each, something like the (Gold upgraded) Laboratory of Monstrosities would be a pretty big deal to help the treasury in a Age that's struggling. (90 Guild Goods x 80 players) x 7 days = 50,400 goods injected per week, plus it'd create goods for that Age which could then be donated manually.
Another use would be for Oceanic and Arctic Future goods production. Because the game requires
Orichalcum and
Promethium in every Age after they're introduced players end up keeping grossly outdated buildings just to make sure they never run out of goods to keep producing the Orichalcum and Promethium without relying on the Space Carrier. The Space Carrier gets worse each subsequent Space Age due to it splitting the chances with another Special Good each time another Age is introduced
Alternatively you could use them on something like the Jumpin' Pumpkin or Magical Winter Snow Globe. Buildings with massive requirements of Population or Happiness reduction in higher Ages.
The only thing you'd lose in reducing the Age for Magical Winter Snow Globe is some of the Forge Points. Because you'd still receive the full output of Renovation Kits and One Up Kits it could be worth reducing the Age so instead of taking up 10,300 Population + removing 36,670 Happiness the building only needs 166 Population and removes 1,620 Happiness
Same thing with the Jumpin' Pumpkin. In the Space Ages the coins and supplies are more important to keep coming in, therefore enthusiastic cities have some value. Keeping the Pumpkin in Titan would require 20,196 Population and remove 24,510 Happiness. Sending it back to Bronze Age would reduce that to 324 Population and remove 1,090 Happiness.
This would be great for Events as then you're still getting army bonuses on that space which is an improvement over blacksmiths while reducing the need for population if you don't have enough to place down more pumpkins
