Slowing is a side-effect of using the One Power that causes the rate of aging to be decreased. It affects both men and women who touch the True Source regularly over an extended period of time, usually taking five years or more of channeling before the effect becomes visible. Slowing manifests physically as a reduction in the effects of time on the body, so people who are over a hundred years old can appear still young. The lifespan of channelers is increased drastically as a result of this, with examples in the Third Age of women over four hundred years old and people in the Age of Legends who are known to have lived much longer - Mesaana for example claims to be over three hundred years old and still considered 'just into her middle years' by Age of Legends standards.
In modern times, slowing is most widely seen in Aes Sedai, many of whom are well over two hundred years old. The oldest Aes Sedai is Cadsuane Melaidhrin who has lived for almost three hundred years - she was also until recently the most powerful sister the order had seen in a thousand years, suggesting that slowing has a greater impact for stronger channelers. Aes Sedai also have an 'ageless' look to their faces that was thought until recently to be a result of slowing but was in fact discovered to be a side effect of the Oath Rod that Aes Sedai swear on upon being raised to full sisters. The Kin coming out into the open also revealed women such as Reanne Corly who are far older than the oldest Aes Sedai yet still do not appear to be extremely old, proving that the Oath Rod also hinders slowing in some way.