Complex sentence: When Cavendish was 12 he inherited a fortune from his deceased father William, but after leaving school at age 17, for the next 8 years or so he spent most of it on luxurious living.
Split into simpler sentences: When Cavendish was 12 he inherited a fortune from his deceased father William. After leaving school at age 17, he spent most of the next 8 years or so on luxurious living.