The “Harry Potter” cast has come of age before our movie-going eyes {thanks, Warner Bros.} When the “Harry Potter” film series is completed, its three young stars — Daniel Radcliffe, Emma Watson and Rupert Grint — will have inhabited J.K. Rowling’s universe for half their lives. Radcliffe, who turns 20 on July 23, was 11 when he was cast as the boy wizard for the series’ 2001 debut. Watson, 19, was 10 when she auditioned for Hermione Granger. Grint, the eldest, is 20. “I’ve probably bee