So what I came up with is "I understand that at best, this demon realm has lived to the extent of 400 years is still considered quite young...?" I added a question mark at the end because there was a "か。。"
The core of the sentence is ～生きた程度では…を理解し切るには若すぎる; "it's too young/short to live~ to fully understand...".
たかだか modifies 四百年.
広さ refers to the extend of the demon realm, not the length of life.
ということか = I've heard that..?
So it would be.. "So I heard that as high as 400 years old, it is still too young to fully understand the full extent of the demon realm"