Learn Turkish: Öğretmen, yaştaş çocukları aynı gruba koydu. - The teacher put the peer children into the same group.

The suffix -DAş indicates sharing a common bond or attribute. When added to yaş (age), it creates yaştaş, meaning people who share the same age, or "peers." Notice that because yaş ends in a voiceless consonant (ş), the suffix changes to -taş rather than -daş to maintain consonant harmony.

Is yaştaş always used as an adjective, like it is here with çocukları?

Not always. Here it acts as an adjective modifying çocukları (peer children / children of the same age). However, words formed with the -DAş suffix are frequently used as standalone nouns. For example, you could just say Onlar yaştaş (They are peers).

Why does the word grup change to gruba?

The verb koymak (to put) requires the dative case to show where something is being put. When we add the dative suffix -a to grup, the final p softens to a b due to consonant mutation. While some foreign loanwords in Turkish resist this softening, grup is one that regularly changes to gruba.

Why is it çocukları instead of just çocuklar?