Take the single variable that best differenciates your target audience from other people’s, and use that as the thing to focus on.
The more you understand the people you are talking to, the more relevant and persuasive your advertising will be. In the industry, the target group is defined through extensive market research. The audience can be general or specific.
But remember, an ad is a conversation between two people (brand and consumer), so the more specific the target group, the easier it is to know exactly who you are talking to, and the easier it is to generate ideas.