The Child of Music was the star of the Concert of the Most Beautiful Music since he was around six years old. He played the lassimater, inside which was trapped a small creature whom he cared about deeply. As the concert took place every ten years, for the following performance he was no longer a child, but a teenager, and then he was in his twenties.
The Tenth Doctor and Donna Noble attended his concert in its 320th year. He was now an old man, with white hair and a liver-spotted scalp. The creature had kept him alive by passing on some of its own life energy, as it couldn't bear to live so long without the only person it cared about. As he afterwards told the Doctor in the corridor, the Child of Music wanted nothing more than to die.
The Doctor returned soon afterwards, and successfully freed the creature from the lassimater, destroying the instrument in the process. The creature let out its final song, and, with his final breath, the Child of Music said to the Doctor, "Thank you." (PROSE: Most Beautiful Music)