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.