I think all the candidates age from the grind, the pressure. I can't imagine doing that, even for a few days, much less months, even years. And the Presidency ages the best of them, a LOT.
I do think that Clinton does look well in his past few public appearances. As for Kerry, I think it could be just not having time in the schedule for the treatments and I do think they are transitory at best. Appearance wise, I don't really care. And in actuality, I think they both look great, given the grind they are under.