The target audience of your product is all potential users who need it at the very least in theory and who can use it physically. Read
why it's important to consider your target audience broadly.
Describe your target audience in terms of:
— their key needs
Demographics are important when you search for respondents. As you know preferences are strongly influenced by demographics :). Just looking at geography we know that when working in different countries we should always test in each country separately, because the market situation varies greatly from country to country. You can determine whether your TA has a need either by asking a direct question "do you have a need for XXX in the last XX", or through behavior "did you do XX in the last XX".
For example, you are making an app for generating recipes depending on what is available in your fridge. The UK is the market you want to research. Then the description of your target audience might look like this:
— Male/Female 20−50 years old.
— UK national representative* sample.
— Those who have been cooking at home for the last month.*Nationally representative sample means that the population of interest is the entire population of the country and that the sample should reflect this in its structure. E.g. the numbers of men vs. women will match the proportions naturally falling out in the real world, the percentage in each age group or each region will exactly match the population etc.
If you want to interview your current users, you don't usually need to limit the target audience any further. However, depending on your objectives you may want to select particular users. For example, you may want to interview your "heavy" users specifically to find out who they are exactly and clarify their profile and preferences. Or vice versa, you may want to talk to your "lapsed" users to understand why they stopped using your product.