Complex sentence: He began to train as a painter at a relatively late age, probably 18 or 20, so around 1503-05, becoming a pupil of Giovanni Bellini and afterwards of Giorgione, both of whose influence is apparent in his works.
Split into simpler sentences: He began to train as a painter at a relatively late age, probably 18 or 20, so around 1503-05. He became a pupil of Giovanni Bellini and afterwards of Giorgione, both of whose influence is apparent in his works.