It's not an idiom, and you need to put 'the' before 'legal'.
Retired English Teacher
Let's see the idiom 'attain/reach legal age'. If 'age' is followed by 'of', there is often an article 'the' before 'legal', for example, 'reach the legal age of adulthood.'
My question is, if 'age' is not followed by 'of', do I still have to put 'the' before 'legal'?
In the sentence below, which is correct, A or B, or which one is more correct? Thank you a lot.
"Seeing that your daughter has reached 'A. legal B. the legal' age, I don't know how you can stop her from marrying me."
In AmE, "legal age" without "the" would be correct.
https://www.google.com/search?q=%22has+reached+legal+age%22&oq=%22has+rea ched+legal+age%22&aqs=chrome..69i57.11265j0j7&sour ceid=chrome&es_sm=93&ie=UTF-8
If 'legal age' is one of the stages of a person's life like adolescent, puberty, adulthood, middle age etc., I think 'the' should not be required.
However, we also refer to it as 'the age of consent'.
Not a teacher
It is the "the age of consent" because of the "of consent" part. "Legal age" refers to the age at which one can legally do something. It does not refer to adolescence, puberty, middle age, etc. One can be of legal age to drink alcohol, to vote, to marry, etc.