The stanzas are from a parody of Robert Southey's poem "The Old Man's Comforts and How He Gained Them", in Alice from Lewis Carroll. First published in 1799, it has been parodied but mostly forgotten and only the parody remembered.
Carroll's parody gives Father William an eccentric and great vitality misunderstood by his questioner.
The line, “allow me to sell you a couple?” suggest that father william thinks "he is more fit than the young man" (letter C), since he can't understand why Father William is much better and still on form in many things he was not supposed to be because of his age and fat body.