There are two foes in the film “Nyad”: nature and age. It is no spoiler to say that Annette Bening and Jodie Foster achieve a conquest over those opponents, not in some trite, scripted or airbrushed way but with the sinews and cartilage of real athletes. Every now and then a cultural moment comes along that exposes how severely and artificially we continue to limit the conceptual range of female ideals, and the cannonball biceps of these actresses in their 60s constitute a significant one. Sun-scorched, straw-haired, scored with tendons, they are glorious.