Though he is burying a son, he does not explicitly say how old the son was. He instead uses imagery to let the reader know that the son he is burying was at a very tender age. This is seen in the use of words such as “Wrapping in your roots a lock of hair, a piece of an infant's birth cord/All that remains above earth of a first-born son” (13-14). At this juncture, the reader finds out that the speaker is performing a venerating symbolic burial of not just an infant son but a first born son for that matter.
While