If we assume that these two couples are in similar financial circumstances (in terms of the housing they can afford), we can say that the elderly couple will generally be looking for simpler housing that is easier to live in and care for while the young couple will be looking for something larger that will have enough space for their growing family.
An elderly couple may well be looking for something small. They might not want to expend the energy needed to maintain a larger house with a yard. They might want to have a home that does not have any steps inside or outside so as to make it easier for them to get around.
By contrast, the young couple has no such concerns. What they want is a place that will give them a good quality of life as their family grows. They will want, above all, want room for their child to play. They will also put more consideration into the issue of schools. They will want to make sure that their home is in a place with easy access to good schools.
These are some of the differences that we would see in the housing needs of families in such different circumstances.