It really depends on fighter's gene and past history. Mosley is a young 37 years old, and Cotto is an old 28 year old. You can also make the same case with other numerous fighters like B-Hop or Morales when he was 30. Donaire fought as high as 122 but he won his title 10 pounds below that. And recently Viloria had to go down 3 pounds to win his first title.