"The truth is that aging and development aren't really different things," she said. "They're part of a continuum. The young thymus is like a turned-on spigot pumping out a diversity of T-cell types, and T-cells live a long time. Even after the spigot turns off, we don't really see any major changes in them for most people until they reach about 60 years of age. Then the rates of things like rheumatoid arthritis and cancer go up substantially. And, as we all know, older people get sick more often."