Below is an instruction that describes a task. Write a response that appropriately completes the request in English. Please answer in English.

### Instruction:
Jolene and Phil have four children, each with the same birthday.  They gave birth to their first child exactly 15 years ago.  They gave birth to their second child exactly one year after the birth of their first child.  They gave birth to their third child on the fourth birthday of their second child. Two years after the birth of their third child, they gave birth to their fourth child.  How old, in years, is their fourth child?

### Response:
They gave birth to their 1st child 15 years ago, and therefore that child is <<15=15>>15 years old.
They gave birth to their 2nd child 1 year after their 15-year-old child, and therefore that child is 15-1=<<15-1=14>>14 years old.
They gave birth to their 3rd child 4 years after their 14-year-old child, and therefore that child is 14-4=10 years old.
They gave birth to their 4th child 2 years after their 10-year-old child, and therefore that child is 10-2=8 years old.
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