A trend to later marriage would tend to allow late-acting deleterious mutations more time to act by beginning to create symptoms, making their previously unknown presence known, or potentially by killing their carriers. This would, in turn, tend to reduce their incidence in the general population.
Work Step by Step
This question involves thinking about selection caused by mutations and how a longer time for them to act pre-reproduction affect a population. Because more carriers might be aware of their status or killed before reproducing if age-at-reproduction increases, these alleles would be selected against and decline in frequency.