The title of "Epitaph for an Old Woman" is key to our understanding of it. First, it tells us that this poem is meant to be an epitaph. An epitaph is a short text that is either printed on a gravestone or tomb. So, that explains how short this poem is.
Yet, we can guess that this is not the type of epitaph that will be read for the woman's family, or printed on her gravestone, because there is still some background information in the poem. For example, it would be weird to print "They buried her in the family tomb" on a gravestone. So, though this poem is labeled as an epitaph, it's really more than that. It's, well… a poem.
In addition to giving us information about the form of the poem, the title lets us know who's being buried. She's an old woman. She doesn't have any name or specific information in this poem. We wouldn't even know that she was old if it weren't for the title. Maybe Paz knew her, and maybe he didn't. She could have been his mother, or just a woman whose funeral he happened to drive by.
Because there's no specific identifying information, she could be any woman, or, really, any person, who has come to the end of her life. The title tells us that this poem is an epitaph for an old woman, but it's also a universal statement on the nature of life and death.