Complex sentence: Young people from 12 to 24 years cause more litter than the average (Dutch or Belgian) person; only 18% of people who regularly cause litter were 50 years of age or older.
Split into simpler sentences: Young people from 12 to 24 years cause more litter than the average (Dutch or Belgian) person. Eighteen percent of people who regularly cause litter were 50 years of age or older.