"year of age" is correct and usable in written English.
Typically, you would use the phrase "year of age" to indicate the approximate age of someone or something. For example, "The cat is three years of age."
They don't start talking until about a year of age.
Starting at around a year of age, 10 months to a year of age, you see a very different pattern.
Half the dogs at any one time were less than 1 year of age.
The highest frequency of biting was reported for dogs less than 1 year of age.
To quantify outcomes of infants (<1 year of age) diagnosed with pulmonary vein stenosis (PVS).
Tracheotomy is most commonly performed in children who are less than 1 year of age.
Cutaneous diseases were more frequent in horses that were more than 1 year of age.
The Bayley Scales of Infant Development were administered at 1 year of age.
Foals and calves have microcytic erythrocytes until one year of age.
Similarly, immunoglobin concentrations are lower in animals under one year of age.
Of the deaths, 39,500 were infants under 1 year of age.
