Consider the civ bios on the official website. They all describe span of time, with the civ being represented by an evolving culture as you age up.
English
Age of Empires IV has you build up the English civilization across four eras: the Anglo-Saxon, Anglo-Norman, English Gothic, and Tudor eras. Each Age has its distinct style from architecture to armor, with populations speaking Old English in the Dark Age and evolving to Early Modern English by the Imperial Age.
Rus
Age of Empires IV allows you to experience the Rus civilization through four Ages portraying distinct moments in history: in Age I, traditional Slavic with Norman and Norse influences; in Age II, the Rus’ Golden Age with Byzantine influence; in Age III, the fall of Mongol influence; and in Age IV, the rise of the Muscovy Duchy.
Chinese
The engineering prowess of the Chinese dominated the world for centuries, a trait present in Age of Empires IV. You live through history as Chinese culture, strength, and innovations created ripples across Eurasia, growing your empire as you move through vibrant Tang, Song, Yuan, and Ming Dynasties. Your units speak Mandarin Chinese, a Sinitic language originating in North China.
I think it’s definitely going to keep a heavy viking theme in dark age and feudal age. Where does that leave us past the 11th century, when vikings no longer existed?
What if it was just the viking age?
It would be a significant tonal shift, no other base civilization is represented in this way. Malian is an outlier, ranging from 11th - 16th century. A true viking civ, based on the viking age, would be limited to 8th - 11th century.
What if the design of the civ lines up with the other 12, and it continues to the rennaissance?
It would still be reasonable to see it as a viking civ (it could easily be plastered all over the bio page and unit mechanics), but use the name that we use for the people across the entire time span (Norse, Scandinavian, etc). It makes no sense to call people vikings after they no longer existed.
In my opinion, the events after the viking age are just as important to viking identity as the events before it. A big part of viking culture was finding a place within the world rather than cutting through it. Once they established wealth and consolidated power they had a more stable society that no longer benefited from piracy.
That might mean an ingame mechanic for choosing between cultures/religions (like the Japanese Shinto/Buddhism landmark). It would also leave room for the Kalmar Union as the imperial age representation, which fielded handcannons and field cannons starting in the late 14th century.