Marriage is full of responsibilities, more so as mankind degenerated into errors and the associated difficulties.  Therefore, marriage is not a business for teenagers, but of people who have some substantial measure of maturity of mind and body – men and women (not boys and girls) (cons. 1 Cor. 7:1-2). 

This is consolidated by the fact that Adam and Eve were created mature human beings (not teenagers), before they were brought together as husband and wife (Gen. 1:26-28; 2:18, 21-24).