A couple of papers to compare and contrast:
While the data supporting a role for melatonin in forestalling aging and prolonging life span per se is not compelling, the findings related to melatonin's ability to reduce the severity of a variety of age-related diseases that have as their basis free radical damage is convincing.
Melatonin has been shown to possess antioxidant properties and to reduce oxidant events in brain aging. .... We found [that a number of] mitochondrial parameters were significantly altered with aging, and that melatonin treatment completely prevented these age-related alterations. These effects appear to be due, at least in part, to melatonin's ability to preserve the content and structural integrity of cardiolipin molecules, which play a pivotal role in mitochondrial bioenergetics.
Which is interesting to say the least; I would have lumped melatonin in with all the other antioxidant supplements - just because a chemical happens to affect some aspects of your biochemistry doesn't mean that ingesting it is going to have any positive benefit.
I have to wonder at what complexity is hidden here: a mechanism completely prevents alterations in mitochondrial parameters, and yet doesn't do anything for life span? Compare that with antioxidant chemicals targeted directly to mitochondria, which lead to significant extensions of healthy life. Mitochondria are complex objects, and (a) the state of their membranes, (b) the working of their inner processing mechanisms, and (c) the effects they have on their cell are not linked in straightforward ways.