The original question was essentially "HOW can someone's city be in one age, and have lots of GB's from higher level ages?" Here is HOW to plant the best GB's in the game, even if you're in Iron Age:
Most players get some GB's eventually... whatever falls in their lap. But why settle for less? With strategy and guild teamwork, you can get whatever advanced-age GB you want! Just decide on the GB’s you want to target planting, then…
TAKE 3 STEPS To Get BP's and Plant the GB’s you Want
1) Start targeting your Aid to get the GB prints you're after. You're going to daily visit the cities of everyone in the neighborhood, in the guild, and in your friend's list... everyone who has a town hall of at least the Age of the GB you're wanting.
So for example, instead of just clicking on a friend's AID button, you visit their city and activate the Motivate (or Polish) button, then hover over buildings or anything with the bouncing star to see what it says the Age of that building is. Once you find a building/structure of the same Age as the GB you want, then click on it. Rinse and repeat. It's a bit time consuming, but it works. (You have the same odds as usual of getting a BP award from aiding, but you're taking advantage of how FoE constrains that awarded BP to being from the same Age/Era as the building/structure that you Motivated or Polished.)
(You can also send Friend invitations to players of one age beyond the GB you want prints for, giving you more chances in your Aiding.)
2) In the main guild chat, ask for suggestions/help for how to get BP's and plant that building. They'll guide you to the best way(s) to swap FP's so that you are putting FP's onto someone else's GB that is the same GB you want. When their GB(s) levels up, you will be awarded one or more BP's of that GB depending on what level and which of the top 5 contributor slots that you're in. Depending on the building's value to the guild, like the Arc or CdM for example, they might provide more aggressive help in GB leveling, to help you collect prints faster.
3) While you are working on getting BP's, also work on getting the required Goods to break ground on your GB. The higher the Age/Era of the GB, the more goods are required and the higher the age of the goods. Everyone knows that it's a challenge to get GB's of much higher ages, and that's where one or more of the seasoned, caring players might offer you a great deal on swapping goods to get there. Don't go on the out-of-guild market unless you have to, because the goods usually cost more.