Last we saw Rocky — in 1990's version No. 5 — the boxer was broke and brain-damaged. So how to explain the recent widower (yes, Adrian's passed on) putting on his gloves again for Dec. 22's Rocky Balboa? ''He'll be fine,'' offers Milo Ventimiglia, 29, who plays Rocky Jr. But even more remarkable than Rocky's ability to keep fighting the good fight is the age-defying fortitude of the movie's writer-director-star, Sylvester Stallone, 60. ''To watch him,'' Ventimiglia says, ''was an amazing thing to behold.''