The question stem is saying that since the two grous are different there is a need of a distinct psychology for the older age group , but B says that since each member of the second age grou (ie.old age grp) has already been a member of the first age group that is- the previous age group , then the two should not be treated differently ...and thus trying to say that the psychology that works for one will also work for the other one ..... I hope this helps!!!!
Whenever two groups of people are so related to each other that any member of the second group must previously have been a member of the first, people in the first group should not be regarded simply as deviant members of the second group.