The target audience is the specific group of people the work must move — defined by behaviour and mindset, not only by demographics.
Age, income and location describe a population. They rarely describe a motivation. The most useful audience definitions in a brief combine a demographic frame with a behavioural one: what they do today, what they believe, and what would have to change.
"Everyone" is not a target audience. A brief that names everyone produces work that moves no one.