Unstable Couples do not Give Birth

In order for offspring to survive, they need to be born in a safe, nurturing a stable environment. Lack of such environment would make the bringing up of new offspring non-compliant with the laws of nature.

If a man and woman get married but are living an unstable life, due to lack of stable work for the husband or constant travel of the husband away from his wife, they might find themselves unable to have any children even though medically they are perfectly capable of having babies. In such case, in order for them to have a higher probability of getting children, they must attempt to make their lifestyle more stable such as be securing a stable job by the husband or for him to travel less.

Real Reason Behind 18+ Marriage Law

The longer the average age of humans the longer is the age of each generation. The younger the age of marriage the more overlapping that takes place between generations. A child may normally see his grandparents in some communities while in others it is common for a child to see his great grandparents and still in others to see his great great grandparents! When life is longer and age of marriage is less, more generations tend to overlap.

As more generations overlap, populations tend to increase rapidly. Restricting age of marriage to be 18+ tends to cap this a bit. Enforcing such rule on developing countries too assists in restricting their high population growth rate.

Scientifically speaking, some people do not mature physically and/or emotionally even after they are 18 years old while others mature both physically and emotionally way before they are 18 years old. The arbitrary marriage age of 18 years, even if taken as an average of maturity, is just an easy way for law makers to have a one-size-fits-all law that can be easily enforced notwithstanding its inaccuracy and nonconformity to the sciences of biology and psychology.