AGEs are biochemicals formed continuously under normal circumstances, but more rapidly under a variety of stresses, especially oxidative stress and hyperglycemia.  They serve as markers of stress and act as toxins themselves.  Pentosidine is typical of the class, except that it fluoresces, which allows it to be seen and measured easily.  Because it is well characterized, it is often studied to provide new insight into the biochemistry of AGE compounds in general.