Complex sentence: Little is known of his painting career prior to his first documented commission in 1278, when, at the age of 23, he was hired to paint 12 wooden panels to cover government documents for The Nine.
Split into simpler sentences: Little is known of his painting career prior to his first documented commission. In 1278, at the age of 23, he was hired to paint 12 wooden panels to cover government documents for The Nine.