Iniesta hasn't lost any bit of his genius now that he's 29, and I would rate Iniesta over Ribery any day. And Xavi didn't decline until he was 32. I doubt that it will be any different for Messi.
For Messi (and to some extent Iniesta) his natural talent (which goes beyond "great dribbling skill" or whatever) makes up for a lot of "flaws" - for example his physique, him being not as good an athlete as Ronaldo, you name it. I guess it will make up for him getting older as well.
As for Ronaldo, having the monster physique that he has, I don't see him losing form in the next few years.
For Messi (and to some extent Iniesta) his natural talent (which goes beyond "great dribbling skill" or whatever) makes up for a lot of "flaws" - for example his physique, him being not as good an athlete as Ronaldo, you name it. I guess it will make up for him getting older as well.
As for Ronaldo, having the monster physique that he has, I don't see him losing form in the next few years.