The Kinsa Ear thermometer is intended for use on both adults and children older than 12 months old. It is not recommended for children younger than 12 months because their ear canals haven't developed enough yet to permit accurate readings.
For further reference, the U.S. Mayo Clinic has the following recommendations for digital ear thermometers:
“Digital Ear thermometers are appropriate for infants older than age 6 months, older children and adults [but] digital ear thermometers aren't recommended for newborns.”
Source: Mayo Clinic