Wenn Friedrich, Gustav Adlof, Condé, Cäsar und Alexander keine Theorie gehabt und den Krieg nicht nach Regeln studiert hätten, wie hätten sie denn in einem Alter Armeen commandiren, Bataillen gewinnen und Eroberungen machen können, da einer so wenig als der andere Erfahrungen haben konnte?

Scharnhorst writes, maybe a bit exaggerating, at least in the case of Alexander who took part in campaigns from his youth on:

If Friedrich, Gustav Adolf, Condé, Cäsar und Alexander had had no theory and had not studied the war according to principles, how could they have commanded armies, won battles and made conquests in an age in which the one could have experiences just as little as the other (means: in their young age in which none of them could have practical experiences in warfare) ?

In this phrase, "da" means: "in which, when", and refers to "in einem Alter" = "in an age".

Forgive my imperfect English.