Complex sentence: Dent's spending wave theory posits that young adults spend little within the greater economy, that spending increases while rearing children, peaks as children leave home, and then slows during the last 15 years of working life (48-63).
Split into simpler sentences: Dent's spending wave theory posits that young adults spend little within the greater economy, and spending increases while they rear children. It peaks as children leave home and then slows during the last 15 years of working life (48-63).