It completely depends on the person, their karma, their lives. My mother was 18 when she and my father got married, been together ever since. But some people just don't meet their permanent partner until later in life.
The best time to get married, is when the time is right for the couple. Some will never marry because we are past the Pluto in Libra generation, "changes in regards to marriage and relationships" and don't want xian or government influence in their relationship.
Keeping things scientific and based on statistics is unnatural. A couple should decide their own relationship and look into factors beyond statistics.
