Learn Turkish: Onlar yaşlı öğretmenler.

In Turkish, there is no separate word for 'am', 'is', or 'are'. Instead, you usually add a personal suffix to the end of the sentence. For the 'they' form (onlar), the 'are' suffix is simply the plural suffix -lAr.

Does the '-ler' in öğretmenler mean 'teachers' or 'they are'?

It actually does both jobs here! It makes the noun plural ('teachers'), and it fulfills the role of the 'they are' suffix you are learning in this lesson. Turkish never stacks two plural suffixes together, so you will never see öğretmen-ler-ler.

Why isn't the adjective pluralized as yaşlılar?

Unlike in some other languages, adjectives in Turkish never change to match a plural noun. They always stay in their base, singular form. So it is always yaşlı öğretmenler, never yaşlılar öğretmenler.

Can I use yaşlı to talk about an old object, like an old school?

No, yaşlı is strictly used for living things (people and animals) because it comes from the word yaş, which means 'age'. For old objects or places (like a school, a car, or a book), you must use the word eski.