In the first example, one could equally well have said "Under 1 year old", but the use of the word "age" seems to add precision. In the second example, one could argue that, in common usage, a child may be referred to as "6 years old " for a period of 12 months at any time between the child's 6th & 7th birthdays, so the expression " children 1 ... More @Wikipedia
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