Good question, it depends on what your website or product you are trying to sell is about, you can identify your target audience, for example if it is children's things it will clearly be children between 5 and 10 years old, if it is things for the elderly it will be between 75 and 90 years old, it depends on what you offer and if for example you sell or have a health and beauty or fitness website, clearly your target audience will be people who are interested in having a much healthier life.