Most people are referring to the inherent genetic features of our biology that causes aging. Because we didn't even know such a features existed until relatively recently. Before that was discovered we thought aging was just caused by all the crap life throws as us and it looked like all we had to do was just keep improving things to functionally have humans who live "forever. "So as far what I've heard from scientists, we're at wall in this area because we still have no idea what the vast majority of the genome even does.