Seydlitz was born on 3 February 1721, in Kalkar in the Duchy of Cleves, where his father, Daniel Florian Seydlitz, was a major of Prussian cavalry with the Cuirassier Regiment Markgraf Friedrich Wilhelm of Brandenburg-Schwedt No. 5. In 1726, his father left military service and moved the family to Schwedt, where he became a forestry master in East Prussia; the senior Seydlitz died in 1728, leaving a widow and children in restricted financial circumstances.[Note 2] Limited schooling was available to young Seydlitz; sources differ whether he knew how to speak and write in French, the lingua franca of Frederick the Great's Court. One biographer, Bernhard von Poten, maintained that his German was good, and if he knew French, he preferred German and wrote it with a "fine, firm hand, unusually correct, in well-formed sentences and with apt expression," and he knew enough Latin to express himself well. His future sovereign, Frederick, always addressed him in German.
By Seydlitz's seventh year, he could ride a horse well, raced with older boys, and he was, by most accounts, a wild and high-spirited child. At the age of fourteen he went as a page to the court of the Margrave Frederick Wilhelm of Brandenburg-Schwedt, who had been his father's colonel. The Margrave was a grandson of the Great Elector, and a nephew to both Frederick I of Prussia and Leopold of Anhalt Dessau. Himself a reckless man, the "Mad" Margrave inspired in young Seydlitz a passion for feats of daredevil horsemanship. Seydlitz did not limit this passion to horses: the Margrave once dared him to ride a wild stag, which he did. Seydlitz became a skilled horseman, and many stories tell of his feats, the best known of which involved riding between the sails of a windmill in full swing. Seydlitz remained in his position as a page to the Margrave until King Frederick William appointed him as cornet in the Margrave's Cuirassier Regiment No. 5 (his father's old regiment) on 13 February 1740.