This strengthens the conclusion. If there had been four hundred students and only four older people, then students would actually have a lower overall rate of forgetting, and the conclusion would seem improperly drawn. But since there were the same number of students and older people, the conclusion seems stronger.

This isn't helpful at all. The study is about when people remembered to make the calls, which we already know was recorded automatically. It doesn't matter who answered the calls, if anyone answered at all.

This establishes that there was a significant age difference between the two groups, and they didn't overlap. This makes the study seem more reliable.

This defends against a potential weakener: that the two groups had unequal access to phones. By ruling out that possibility, the study seems more reliable, and the conclusion more likely to be valid.

This helps strengthen the conclusion. If the college students had received their instructions a week before the test, versus the older people receiving their instructions the day before, that would make the study's results less useful. But this establishes that both groups had the same amount of time where they had to remember to make the call.