It is difficult to provide an overview of Buddhism's approach to the stages of human life. This is because in most cultures the observances of the life cycle were developed before Buddhism entered the region. Buddhism forms more of an additional layer or veneer on top of older rites than the rites' cores. Often, Buddhist monks do not even officiate at these ceremonies; the rites are instead performed by some sort of lay "folk priest." The monk's role may consist of doing nothing more than saying a few blessings. In addition, the local character of these ceremonies causes them to vary widely from culture to culture and from country to country. So Buddhism usually plays a secondary role in the rites of life passages, and a role that shows no consistency across the many cultures to which Buddhism has migrated.
Death stands out as the one area where Buddhism plays a large role and where the monks are most intimately involved. This is because it is death that most obviously marks the suffering of samsara. It is a time for discussing the impermanence of life, the journey towards rebirth, and the importance of merit. Indeed, funerals, some of which may last for many days, include rites transferring merit to the dead, ordinations of monks, and funeral processions.
In Tibetan Buddhism (Vajrayana), it is believed that it is possible for a person to attain nirvana between their death and their rebirth. The Tibetan Book of the Dead provides an elaborate description of the stages through which a soul passes on the way towards rebirth. The stages parallel the three aspects of the Buddha, beginning with his ultimate cosmic character and then, as the soul gets closer to rebirth, works towards the earthly character of his appearance on this earth. Passages from the Book of the Dead will be read to the deceased, helping them recognize the stages through which they are passing and instructing them in the choices that will lead to nirvana rather than rebirth.
In southeast Asia, Theravada Buddhism also plays a key role in the passage of young men from adolescence to adulthood. These young men will temporarily enter the sangha for a period of time ranging anywhere from the three-month period of the Rain Retreat to a year or more. While in the monastery, they will learn about Buddhism, undertake to follow the Ten Precepts of the monks and other regulations for the sangha, and essentially live the life of a permanent monk. When they complete their time, they will be considered adults by the community and be eligible for marriage.