Old dorgs nuss their grudges, but yung purps fite, and then frolick.

[Old dogs nurse their grudges, but young pups fight and then frolic.]

Josh Billings (1818-1885) American humorist, aphorist [pseud. of Henry Wheeler Shaw]

Josh Billings’ Farmer’s Allminax, 1870-06 (1870 ed.)
    (Source)

This aphorism shows up again in Everybody's Friend, Or; Josh Billing's Encyclopedia and Proverbial Philosophy of Wit and Humor, ch. 156 "Affurisms: Embers on the Harth" (1874), with slight spelling changes:

Old dorgs nuss their grudges, but yung pupps fight and then frolik.