Tennis has long been associated with high-profile stars and young phenoms, but a growing number of active senior tennis players continue to compete long after athletes in other sports have hung up their cleats. For players age 80 and older, this is the world of Ultra Senior tennis, where octogenarians train like young pros and roam the highways like itinerant rock bands. The most obsessively driven players even look forward to getting older, because every five years they “age up” into the next tournament category, allowing them to have relative youth on their side — at least for a precious little while. “Gold Balls” tells the uplifting story of five seniors who barnstorm the U.S. in relentless pursuit of a national championship. The documentary accompanies the quintet — the oldest is 94! — on a cross-country odyssey filled with obstacles large and small, ranging from mundane travel challenges and aging body parts to the ultimate opponent: mortality. A film about the indomitable human spirit, “Gold Balls” explores universal questions about aging and forcefully demonstrates the youth-maintaining power of pursuing a goal.