Political or religious views can evolve.
Ideas about children can shift.
Social circles change.
People may move across the country, return to school, change careers or discover that a lifestyle that once seemed attractive no longer fits them.
None of those changes automatically damages a relationship.
Couples sometimes grow together extraordinarily well.
But when two people marry before either has developed a stable adult identity, there is a greater possibility that they will develop in different directions.
Someone marrying at 30 usually has more information about who they are than they did at 18.
That does not necessarily make them wiser in every situation, but it can make an enormous decision such as marriage more informed.
Emotional Maturity Can Make Conflict Easier to Manage
Marriage requires much more than attraction.
Long-term relationships involve disagreements about money, careers, sex, family responsibilities, children, household labor, relatives and countless everyday decisions.
How couples respond to those disagreements can be crucial.
A person who has had several additional years of adult life may have developed better communication skills and greater emotional regulation.
They may be more capable of recognizing that anger does not automatically mean a relationship is failing.
They may be more comfortable apologizing.
They may better understand the difference between a temporary disagreement and a fundamental incompatibility.
They may also be more willing to establish boundaries rather than expecting a partner to instinctively understand every emotional need.
None of these skills appears automatically on a particular birthday. Some 21-year-olds are exceptionally mature, while some people decades older continue to struggle with communication and conflict.
But adulthood provides opportunities to develop those skills through friendships, employment, relationships and other experiences.
Waiting before marriage can therefore mean entering the relationship with a larger emotional toolkit.
Financial Stability Can Remove a Major Source of Pressure
Money is another important part of the equation.
Very young couples are often beginning adult life at the same time they are beginning married life.
They may be completing college, entering the labor market, paying student loans, trying to afford housing and establishing financial independence simultaneously.
That combination can be difficult.
Financial instability does not mean a marriage will fail. Many couples build successful lives while starting with almost nothing.
But financial pressure can create additional conflict in relationships that are already adjusting to the responsibilities of adulthood.
People marrying later may have had more time to establish careers, build savings and understand their own financial habits.
They may know whether they tend to spend or save.
They may have learned how much debt they are comfortable carrying.
They may have a clearer idea of their long-term career prospects.