Broadly speaking, there is the expectation in today's society that voluntary marriage (i.e., marriage where the girl is not pregnant) will only take place after both partners have completed what is foreseen to be their terminal education or vocational training. When a girl gets married at 18 or below, it is generally a concession by her that she will never do anything other than be a housewife and mother (which is the noblest of vocations for a laywoman), or some kind of menial job in retail or food service. A boy who marries below the age of 21 or 22, unless he has some kind of potentially lucrative vocational training (plumber, welder, electrician, etc.), is conceding that he will never be able to support a larger family, and might well consider deferring marriage until he is better established in life, assuming that ever happens. Our society basically treats anyone under 30 as a de facto minor whose life is devoted either to serious career development on the one hand, or pursuit of endless fun and "having one's space and freedom" while they "find themselves", on the other hand. The military would be an exception to this.
One thing I think also needs to be brought up, is that both males and females (especially the latter) mature physically so much earlier than in times past, yet they are treated (in this regard) as children for many years, years during which they are physically mature, yet generally speaking, unsuited for marriage otherwise. This is dangerous to purity --- they are surely physically equipped for marriage, and have all the accompanying desires, yet cannot act on them without committing the sin of fornication. To remain chaste and virginal for over a decade, when desires are at their peak, requires heroic virtue and self-denial.