Learn Swedish: Vi undrade varför åldern var en hemlighet. - We wondered why the age was a secret.

Yes! Åldern means "the age" (from the base noun en ålder). You will notice it shares a root with äldre (older) and äldst (oldest). While the basic adjective for "old" is gammal, it completely changes its stem in the comparative forms, linking back to this same root for age.

Why is the word order varför åldern var instead of varför var åldern?

If this were a direct question, the verb would come right after the question word: Varför var åldern en hemlighet? (Why was the age a secret?). However, in this sentence, varför introduces a subordinate clause (an indirect question). In Swedish subordinate clauses, the subject always comes before the verb, so we say ... varför åldern var ....

Is there a trick to remembering that hemlighet is an en word?

Yes! Nouns ending in the suffix -het (which is similar to English endings like "-ness" or "-ity") are almost always en words. So it is en hemlighet (a secret) and the definite form is hemligheten (the secret).

How do I find the present tense of undrade?