Ways to avoid repetitions, ranked from best to worst

1. Ellipsis*
2. Reference, substitution
3. Shell nouns
4. Nothing wrong with actual repetitions
5. Synonyms

In the quiz above, can you write, say, "in the next three age groups"? Of course, you can. It's not a wrong answer. But it's not the best answer.

First, ellipsis* is just more elegant. Second, think about the big picture. It's likely that you will have to write "age group" (and synonyms) many more times in your answer. (This specific sentence was taken from my answer to an IELTS task with seven age groups and two genders.) Even if you do find many ways to paraphrase "age group," they will still attract attention and make the answer bulky.

* Ellipsis = the omission from speech or writing of a word or words that are superfluous or able to be understood from contextual clues.

So, if you see an opportunity for ellipsis, seize it.