1 year old vs. 1 year of age - is one of them the "correct" form?

I know that both forms (1 year old and 1 year of age) exist, but when I tried to find some rule about usage, I couldn't (there are several forum threads about it, such as this and that, but they are not really helpful as they do not give ANY direction regarding the difference between the forms (if any)).

I need to use one* of these phrases in a formal context (drug leaflet). Currently the text says: "The medicine is not intended for babies under 1 year of age. For children 1 to 6 years old, the medicine is dispensed with a doctor's prescription only".
My questions are:

1. Is there any preference to use either form, objectively or contextually?
2. Is there a preference to use one or the other when stating an age range, i.e. would it be preferable to say "children 2-5 years old" over "children 2-5 years of age", or vice versa?

*I believe the same phrase should be used throughout the text.

Solution 1:

Under 1 year of age marks a specific upper limit that is reached on the child's first birthday.

On the other hand, you might refer to a child as "1 year old" at any time between the child's first & second birthdays.

Nevertheless, that does not wholly address the issue for two reasons:

1. 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.

2. 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 to 6 years old" is actually ambiguous. Does it mean:

   • a period of 5 years between the child's 1st & 6th birthdays; or
   • a period of 6 years between the child's 1st & 7th birthdays, during the last year of which the child may commonly be described as being "6 years' old".

Hence it is unclear whether the need for a doctor's prescription ceases on the child's 6th birthday or not until the 7th birthday. But this ambiguity is related not to the usage of the word "age" v. "old", but to the phraseology used. To be clearer the second qualification should have read (for example):
"For children over 1 but under 6 years of age [or 6 years old], ...". With such wording:

• The expression "over 1" is clear because of the preceding qualification.
• The use of the word "under" makes it clear that the upper limit is the child's 6th birthday & not his/her 7th birthday.
• The word "old" could be used instead of "age", but the use of "age" is consistent with the previous sentence and seems to add precision.

In conclusion, (as has been noted in the other answer) the use of "age" implies a legality, but - more critically - the upper limit needs clarification as to whether it is inclusive or exclusive of '6-year olds', preferably by using wording such as "under 6 ...".

In answer to the question "is one of them the “correct” form?", I would say that neither form is correct or incorrect: it depends on both choice & usage, but - even more importantly - clarity is needed when specifying an upper age limit.

Solution 2:

Things that lawyers have a hand in tend to use very precise and immutable language to minimize the possibility of common words changing meaning over time or being interpreted in different ways.

For example, look at gender identity (chosen as an example because it illustrates common usage changing over a short time). Common usage of "Male" and "female" human gender used to refer to the biological definition. Now the terms can mean something totally different, virtually unrelated to biology (i.e., the gender the person views themself as or would prefer to be treated as).

"Old" could similarly come to mean something other than physical age. People already refer to how old someone feels or how old someone acts. Children are referred to in terms of how old they are physically or developmentally. "Old" is a somewhat ambiguous term, and its casual, imprecise use in everyday language makes it the kind of word lawyers prefer not to use.

To minimize the potential for confusion (and liability), legal and medical documents use terms that are as unambiguous as possible. "1 year of age" is less likely than is "1 year old" to be interpreted as referring to something other than chronological age, and is likely to weather the evolution of language better.

In normal conversation, though, "1 year old" would be the typical expression (with the possible exception of someone like a lawyer, for whom the use of precise language is ingrained).