Most studies of marriages of long duration find that compared to younger adults, the elderly are

I believe the answer is: Happier in their marriages

Couples who already lasted until elderly age tend to already experienced a lot of obstacles together, which would naturally increase their appreciation toward one another.
 Couples among young adults on the other hand, tend to be in the early stage of romance/exploration and they also still have a lot of time to find new partner and change the dynamic of their relationship;

It all depends on the individual