In my opinion, forcing people to do something they don't want to do (ie HAVE to move) is not a good idea.
It might keep things more equal but then you are not allowing intelligent, hard working or rich players to stand out.
Communism by another name.
Didn't mean they should
force players to move up, rather that if the player
wants to continue growing or progressing quickly through the ages then they should be required to enter the next group of eras, leaving the previous group behind bar some basic interaction like trading or helping guildmates.
They obviously have the option to stay in an era (or set of eras) for as long as they wish.
Think of it like splitting the entire game into a few parts where you exit one part at a certain point/level/strength and go on to compete alongside players similar to yourself in the next part. You would decide when that was, not the Game.
If each group of eras contains 3-4 ages there is still scope to challenge yourself against those players 3-4 ages ahead of you (or plunder those 3-4 ages below!)
The intelligence and hard work would come about by preparing your city for entering the next phase, especially if. for example, access to much higher era GB's was limited until you were much closer to that GB's era group. That might test some players' skills...