The Kinsa Smart Ear thermometer is FDA-cleared for adults and children of all ages.
For infants under the age of 1, gently tug the ear straight back to straighten the ear canal before taking a temperature reading. For children over the age of 1 and adults, gently tug the ear straight up and back.
Due to the wide variety of shapes, sizes, and growth rates of the ear canals of newborn babies, the Mayo Clinic recommends waiting until babies are at least six months old before using ear thermometers in general. Ear thermometers use an infrared ray to measure the temperature inside the ear, and so earwax or a small, curved ear canal can interfere with the accuracy of a temperature reading.
For more advice, please check out this article from the Mayo Clinic: Thermometer Age Guidelines