Roughly 4 to 10. Reye's syndrome is now rare since Aspirin was found to be the cause rather than the virus for which the Aspirin was given. I have seen it in an eighteen month child so the age is not in a definite range. There are still occaional rare cases of the syndrome where Aspirin was not given so the mechanism is still unknown.
<16y. This disorder emerged in the late 60's, seemed to peak in the late 70's and is now quite rare.Once we understood its link to aspirin, the general effort was to avoid it under age 16 and specifically for flu or chickenpox. We don't understand the real cause or whether stopping aspirin products led to its decline in frequency.Aspirin is occasionally still used in kids for specific disorders.