When a man has reached an old age, and already has grandchildren, then he, and possibly his wife, should retire to live as hermits in the forest.
Ideally, the older person who will retire to the forest will already have a son who will stay at home, since in Hindu tradition the son has the responsibility of lighting the cremation pyre for deceased parents.
In this post-householder stage of life, one will recite the Veda and follow spiritual practices for purification.
Like the student, the older practitioner will beg for food, while practicing austerities to generate spiritual energy or inner heat (tapas). This will allow the older hermit to reach greater spiritual understanding.
Since the older hermit has already fulfilled his societal duty by producing children (especially sons), he can pursue the aim of mokṣa (spiritual freedom, or leaving the everyday world of saṁsāra).
In this āśrama system, Brahmins were the class of people regarded as the ideal models of such practices.