Nadal will be done before 30, Djokovic 31 tops, Murray may age the best of the lot seeing as he neither destroys his knees like Nadal or slides on hardcourts as often as Nole.
instinctively I would say something like that : Murray ageing better than Djokovic ageing better than Nadal, for the reasons you say, and also because Murray maybe loves tennis more and has a steadier attitude to it than Djokovic imo (just a feeling like Ash86's) and has more variety and a better first serve.
But overall I think Djokovic and Murray can last very long on top considering the poor younger generations.
They have a limited shelf life as do all defensive players (except Ferrer but hes some sort of freak).
it's not true (considering "defensive" means "grinders" in this forum), if you look at the lower ranked players who age well, many of them are rather grinders (Russell, Ramirez-Hidalgo, Schuettler, Eschauer or Luczak in the past ...) but having a little bit variety also helps to adapt (see Santoro or even Schuettler and others).