There were people who`s age group was...
but you might also have meant...
There are people who`s age group was...
the people you are speaking about...
``are`` means they are still alive
``were`` means they are dead or you are speaking about them in the past.
``age Group`` becomes considered singular and in the past tense for this particular sentence. You are naming one aspect of all these people.
``Is`` means right now or this very instant; present tense. This is also ok if you are talking about them in there current state.
```was`` is good for describing them how they were.
``were`` and ``are``, are wrong they did not fit this sentence where you put them.