One of the features and characteristics of a living thing is to reproduce and propagate its species. Humans are no exception. All the other living things follow the natural cycle and instinct in this regard.
It is only humans that treat co-habitations a means to satisfy psychological and social pleasure and bliss. That is why human resort to non-natural and even unnatural methods to enhance the individual, psychological and social happiness and well being as they feel.
If we follow the proper natural cycle of generation and regeneration, then it is reasonable kept as twenty five years as one generation. If the son and father or mother and daughter are different in age beyond twenty five years then that is called a generation gap. The parents and children may be in different wolds and may not go hand in hand , but may be at logger heads.
Man or woman, are at their youth's peak when they are around twenty five. The nature needs the best off springs and the ideal period is the prime youth. Wisdom has keep age of twenty five as an optimum age with physical and mental maturity for both man and woman.
So if a girl gets married at or before twenty five and gets her first child soon, her child will be ready to start anew generation by the time this mother is fifty.following same tradition. The grandchild then gets a reasonably healthy grand parent for experienced care and attention and the new mother also gets her support to be a worthy mother.This is true for men also. However as there is a slight gap in mental maturity age between boys and girls, and as male productivity can go more than female, a slightly higher age for men getting married is condoned.
It is also a fact that delayed pregnancies and child births can cause unwelcome issues. Hence parents and concerned elders get worried if the marriage of their daughter is getting delayed.
What I said is just traditional wisdom. However in modern age when there are various other considerations and also facilities and handicaps, and marriage is not for the sole purpose of having offspring, there can be customisation in all these matters.
The present world suffers from the problem of imbalanced birth rate continuity. With less birth rate and more generation gap due to child spacing, there is a disproportion between the old population and the youth and children segment.