Learn Italian: La loro stirpe è molto antica. - Their lineage is very ancient.

The possessive loro (their) is a strict exception to the rule about dropping articles. While you normally drop the definite article before singular family members (like mio padre for my father), loro always requires the definite article, no matter what noun follows it.

Wait, does stirpe (lineage) even count as a family member for that rule?

No, it doesn't! Words that describe collective family groups or concepts—like stirpe (lineage), famiglia (family), or parentela (relatives)—do not follow the "no-article" rule for singular family members. So even if the sentence used sua instead of loro, it would still require the article: la sua stirpe.

Since stirpe ends in -e, how can I remember its gender?

Stirpe is feminine, which is clearly shown here by the article la and the adjective ending in antica. While words ending in -e can be masculine or feminine, nouns ending specifically in -pe (like la volpe for the fox or la rupe for the cliff) are almost always feminine.

Is stirpe a common word in modern everyday Italian?