Learn Polish: Młodość to czas, kiedy szybko dorastamy. - Youth is a time when we grow up quickly.

When equating two nouns in the nominative case (Młodość and czas), Polish typically uses the particle to to mean is. If you wanted to use the verb jest, the second noun would have to change into the instrumental case: Młodość jest czasem.

How is kiedy being used here? Isn't it usually a question word?

Here, kiedy acts as a relative pronoun, linking the noun czas to the clause that describes it. Just like in English, the question word for when doubles as a relative pronoun to mean the time at which.

Is the comma before kiedy mandatory?

Yes. In Polish, you must always place a comma before relative pronouns like kiedy, gdzie, or który when they introduce a new clause.

Why is the verb dorastamy in the imperfective aspect?

Dorastamy comes from the imperfective verb dorastać meaning to grow up. We use the imperfective here because growing up is described as a continuous, ongoing process during youth, rather than a single completed action. The perfective equivalent is dorosnąć.