This is the 481st most frequent Czech word.
věku
The Czech word "věku" means "of age" or "of the age" in English.
The word 'věku' here is used in a prepositional phrase that indicates the age of the girl, serving as a contextual descriptor for her learning capacity.
Dívka v tak mladém věku se učí rychle.
A girl at such a young age learns quickly.
The word 'věku' is implied from its base form 'věk'. In this instance, it's referring to the concept of age in a comparative statement about societal trends.
Věk dožití v této zemi se zvyšuje.
The life expectancy in this country is increasing.
Here, 'věku' is part of the genitive case construction indicating the age of the man at the time of an important life event.