Consider the difference between “of age” and “of a certain age.” The expression “of age” has more than one meaning out of context, but only one meaning in context. The age in “of age” can vary from bar/bat mitzvah age (13) to debutante age (15 or 16) to age of majority (18) to drinking age (21). These are just examples; of course these milestones vary based on culture and law. The important thing to realize is that, in any given context, “of age” is meant to be taken at face value. “You’re not of age” means “you’re not eighteen” at a military recruiting station and “you’re not twenty-one” at a bar. These are statements of fact.
On the other hand, the word “certain” in the phrase “of a certain age” is anything but. When someone says “of a certain age,” they refuse to be specific; instead, they invite the listener to share in the creation of a positive or negative judgment. A person “of a certain age” could be in their 30s, 40s, 50s, 60s– or even 90s! “Of a certain age” reflects the attitude of the speaker, while the phrase “of age” is neutral. “Of age” is a social construct, but “of a certain age” is a social statement.
In short, the expression “of age” is ambiguous but not vague, while the phrase “of a certain age” is vague but not ambiguous. I hope this helps clarify the difference between ambiguity and vague language.
How about you? How do you interpret ambiguous vs. vague language?