Their 1st child was born 15 years ago, and therefore is <<15=15>>15 years old.
Their 2nd child was born 1 year after their 15-year-old child, and therefore is 15-1=<<15-1=14>>14 years old.
Their 3rd child was born 4 years after their 14-year-old child, and therefore is 14-4=10 years old.
Their 4th child was born 2 years after their 10-year-old child, and therefore is 10-2=8 years old.
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