He cannot fight in his late thirties simply for the fact that he's been boxing all his life. He turned pro at age 19 and that's also 11 years of pro experience as well. All that training all those fights take a toll on you. Just look at Erik Morales for example, granted Mayweather never took beatings like Morales. Point is Mayweather is no longer a "fresh fighter", like some other boxers who are in there thirties.
He's a complete fighter in his style. But I consider Chad Dawson to be a complete fighter because he does it all as well, but he's more offenseive minded then Mayweather. I mean there is not one technical flaw in his boxing style.
It's impossible to say. Part of what makes him who he is has to do with his reflexes, which inevitably go away as one ages. You see the trend quite consistently in nearly every fighter. Mosley, De La Hoya, Jones Jr. are prime examples of fighters with phenomenal natural ability, but time started catching up to them. People forget that all three of those guys were just as badass as Mayweather in their primes. Mosley didn't lose a fight until he was 31 years old, and that was against a prime and undefeated Forrest. De La Hoya lost two close fights to prime ATG's in Trinidad and Mosley (before he was 30) and Roy Jones didn't lose really lose until he was 34 (not counting the Griffin DQ). Honestly, how Mayweather does in his early to mid-30's as an older lion against young lions like Cotto, Margarito, Williams etc...will truly define his legacy. Will he at 34 be able to be competitive with a guy considered p4p #1 like De La Hoya was with him? Or step up in weight to a guy like a Bernard Hopkins, who was ranked p4p #1 at the time as well, as De La Hoya did too? Or take on a young beast of a welterweight like Cotto when he's 36 and make a good account of himself like Mosley did? That's when people will truly know how good compared to other fighters he is when he retires.