What moral do the characters sing in the final quartet of Don Pasquale?

In the final quartet of Gaetano Donizetti's opera Don Pasquale, the characters acknowledge the moral of the story: that it is foolish for an old man to marry.1

This conclusion follows the resolution of the plot, in which the elderly Don Pasquale is tricked into a sham marriage by Dr. Malatesta, Norina, and Ernesto. After the ruse is revealed and the young lovers are forgiven, the characters reflect on the folly of Pasquale’s attempt to marry in his later years, summarized in the quartet titled "La morale in tutto questo".1