It is noticeable, first of all, that among the Australian tribes the older men have worked out or fallen into such habits regarding the females that the younger men obtain wives with great difficulty and usually not before waiting a long time.  In fact, Spencer and Gillen, in their invaluable works on the central Australian tribes state that usually a man is married to a woman of another generation than himself: 

The most usual method of obtaining a wife is that which is connected with the well-established custom in accordance with which every woman of the tribe is made Tualcha mura with some man.  The arrangement, which is often a mutual one, is made between two men, and it will be seen that owing to a girl being made Tualcha mura to a boy of her own age the men very frequently have wives much younger than themselves, as the husband and the mother of the wife obtained in this way are usually approximately of the same age.  When it has been agreed upon by two men that the relationship shall be established between their own children, one a boy and the other a girl, the two latter, who are generally of a tender age, are taken to the Erlukwirra, or women’s camp, and here each mother takes the other child and rubs it over with a mixture of fat and red ochre....  This relationship indicates that the man has the right to take as wife the daughter of the woman; she is in fact assigned to him, and this, as a rule, many years before she is born.[217]

It will be noticed that this is in reality a modification of the system of exchanging women, and has an advantage over capture, elopement, and charming (all of which are methods in practice among the same tribes) in the fact that it is of the nature of a business transaction or social agreement, and provokes no bad feeling or retaliation.  It also shows considerable regard on the part of the elders for the young; but practically it is a reluctant admission of a youth to participation in sexual privileges, since marriage is delayed until a girl of his own age has been married and given birth to a girl who in turn has become marriageable.

In the same connection we have the testimony of Curr that

the marriage customs of the blacks result in very ill-assorted unions as regards age; for it is usual to see old men with mere girls as wives, and men in the prime of life married to old widows.  As a rule wives are not obtained by the men until they are at least thirty years of age.  Women have very frequently two husbands during their lifetime, the first older and the second younger than themselves.  Of course, as polygamy is the rule and the men of the tribe exceed the females in number besides, there are always many bachelors in every tribe; but I never heard of a female over sixteen years of age who, prior to the breakdown of aboriginal customs after the coming of the whites, had not a husband.[218]

And Bonwick says: