This Design Spotlight is brought to you by John Hawkins, Lead Designer!
Currently we match players up with players who are within one age of them (up or down). In the past we have tried letting players attacking two ages down, but that led to bad experiences for the lower age players, and not enough loot to make it worth keeping for higher age players.
This philosophy is complete failure. The reason strong players pick on weak players and give them "bad experiences" is because YOU, Nexon, are too cheapskate with your league bonuses. The strong players who wish to farm have no incentive to climb in medals.
I am currently enlightenment age. I have 100% maxed out my gunpowder age before proceeding on. I am currently finding it extremely difficult to climb out of silver league, where the league bonus is pathetically less than 5k of each, and yet, I have seen players who are still medieval age who are in kingdom league. Does it make any sense to you that a midevil player can climb easier than a maxed gunpowder who is now enlightenment?
Limiting players from attacking only one age difference is just plain WRONG. What you should do instead is allow players to attack ANYONE, regardless of age, but, give players a real reason to climb in medals, so the stronger players don't feel it's in their best interest to stay low in medals.
You could also buff the farm/caravan production rate at higher levels so that stronger players would feel weaker players don't have much to offer.