at what age does the speaker suggest that children are ready to be made useful to society through his plan?
Toward the end of the fourth paragraph, the narrator says,
[...] it is exactly at one year old that I propose to provide for [babies] in such a manner as instead of being a charge upon their parents or the parish, or wanting food and raiment for the rest of their lives, they shall on the contrary contribute to the feeding, and partly to the clothing, of many thousands.
Part of the reason he chooses the age of one as the time to implement his plan is that a child, up until that point, can survive on its mother's milk alone. The cost of the child's subsistence, then, is so cheap—he estimates about two shillings for that first year—that the mother will have no trouble coming up with the means to support the baby until the child turns one.
The narrator also believes that a one-year-old child would provide a great deal of meat, considering how little money it would take to raise to that age. However, he completely miscalculates how much a typical child weighs, another way that Swift undermines the speaker's intelligence. He says that a newborn typically weighs "12 pounds" and "in a solar year, if tolerably nursed, increaseth to 28 pounds." Typical newborns do not weigh twelve pounds, and most one-year-old children do not weigh anyhere near twenty-eight pounds. However, because the speaker believes that a child of this age would provide so hearty a portion of food, he sticks with the one-year mark.
Jonathan Swift's satirical essay A Modest Proposal is his attempt to critique Irish public policy and to critique the lack of caring which the rich of his time exhibited toward the poor. Swift's proposal states that poor Irish men and women would stand to gain if they were to sell their children to be eaten by rich men and women. Although intended as a satire, Swift goes to great lengths to develop a logic to his point. He states that children that are a year old would be ideal for this venture, as they would have been well nursed and would have the right body composition for proper cooking and consumption.