Like their muscles, the box-office drawing power of Sylvester Stallone and his action-hero cohorts appears to be atrophying with age: “The Expendables 2,” with Mr. Stallone and a muscular ensemble, was the No. 1 movie in North America over the weekend. But its estimated $28.8 million in ticket sales was 21 percent lower than its series predecessor’s opening-weekend total, after accounting for inflation. Millennium Films spent about $90 million to make “The Expendables 2,” which was distributed by Lionsgate.