MB 11: Age of Thirteen Years - This is before he brings proof that he reached puberty.
MB 12: This is the accepted custom - The custom now is to start wearing Tefilin two to three months in advance and look in the Be'ur Halacha (where it is explained that a child of 12 that is learning Talmud and understands about Tefilin can be trusted to be careful while wearing Tefilin). Someone who speaks but cannot hear, or can hear but cannot speak is obligated to wear Tefilin, but if he cannot hear or speak (he is exempt from wearing Tefilin) but can wear Tefilin if he so desires.
Binyamin Rudman