Just keep in mind that the advantage for u is, u can also attack players in medieval, so 4-5 starring is possible.
lets set the advantages of attacking different ages in a row:
attacking a lower age : 4-5 star victory is easier to gain, will provide you with diamonds if you need them
attacking same age : 2-3 star victory, but you may find more loot, cause the market, mill, farms and caravans can store more. And these players have more expensive upgrades, so there might be more in stock.
attacking higher age : May grant you with more medals per win, as these players are sometimes higher in medal ranking, also more loot to get there.
Being attacked by a higher age player, completely wiping you of the face of your base (I like this sentence) will grant you with a longer peace treaty.
The amount of resources that a player can loot from you is the same, no matter what age they come from.
Also, anybody that wants to get a victory star, will eventually get it, no matter what.
So bottom line, its a bigger advantage to being able to pick your battle from 1 age down to 1 age up, then a disadvantage.
And there will be more bases to choose from at the end of the day (fear the NO OPPONENT message)
I personally believe in every age being able to attack any age, but... it should be less rewarding for a higher age/level base to attack and loot from a lower age/level base.
Believe me, defending your base against any attack is virtually impossible, you will be defeated at some time, the only thing you can do is damage control by protecting your precious resources(market and mill) and your storehouse for trade goods.
placing your TC outside as a gift for medal hunters, will make you keep more resources, placing it inside walls, protected by your defense buildings makes TC snipers skip your base and players with high level/age wipe you out.
So there is your choice of defense, TC in or TC out of your defense.
You cannot place your resource buildings and TC both in a defense position.....