Complex sentence: Mozart would have been between 6 and 8 years of age when he composed these works; hence it is believed by many that it was written down for the boy by his father, Leopold: all four of these early sonatas are preserved in Leopold's handwriting.
Split into simpler sentences: Mozart would have been between 6 and 8 years of age when he composed. All four of these early sonatas are preserved in Leopold's handwriting.