This question is very broad. Monarchies abound all over the world and each has its own unique traits, and laws; beyond that expediency can also play a decisive role.

I do know of one instance where there were two female heirs to the throne and one male younger brother. Both daughters ascended but the male heir never did. King James II of England had two daughters Mary (elder) and Anne (younger), and eventually a younger son (James) as well. In normal circumstances James would have become the heir to the kingdoms however several factors intervened in this.

Firstly James II was Catholic, and through various policies or missteps James II found himself on the wrong side of a revolution (The Glorious Revolution 1688). Secondly there was rumor that the boy James was not a legitimate child, Mary* and Anne would hold this belief (at least publicly) as it provided legitimacy to their own reigns. The revolution brought William of Orange into England. William was married (since 1677) to Mary.

So we are left with a confused situation. The old King James II alive and well in exile in France, his son James with him, and both Catholic. The princesses Mary and Anne in England, and William of Orange in England as well (with an army). The confusing situation led to the following solution: James II was determined to have abdicated the throne, Anne was to voluntarily move her claim down below that of William and become third in line. Mary and William were to rule jointly but in their own right so that the other would rule if either died. Mary died first in 1694 (6 years after the revolution, without child), and William reigned until his death (in 1702), at which point Anne ascended to the throne (and died in 1714). The boy James (died 1766) never ascended and lived his life in exile in France after his fathers (James II) death (though both James II, and the boy James attempted to reclaim the throne). On Anne's death the German Hanoverian prince elector George I ascended to the throne.

In this case despite the fact that there was a King (Jame II) who was alive, and a son (James) the eldest daughter and her husband reigned jointly, then the husband alone, then the other daughter, then a foreign German prince. However in this case both women were of child bearing age Mary died at age 32 in 1694, and Anne ascended at 1702 at age 36, and died in 1714 age 49.

So to the initial question of would an older woman step aside for a younger? Maybe, maybe not. In the case of England securing the Church of England's dominance by insuring a Protestant ruler took precedence over a neat succession.

Source: Marlborough: His Life and Times by Winston Churchill