The answer is simple: when the vines are too young, like those pictured below. Removing the fruit, which is too sparse to be worth picking anyway, directs all the energy toward growing a strong plant capable of producing fine winegrapes for many years. (There are other reasons, but that's for another day.)
We'll let these young vines ripen their first small crop next year, which will be their third leaf. By year five, Mother Nature willing, they should be in full production.