Oh, do you mean demographics? well you need to figure out yourself. If you target health niche, well this should be universal, as a disease can affect both genders, and it can occur at any age.
If you target gaming niche for instance, then expect male people between 18-34 to be mostly interested.
Re countries: USA, Canada, UK and Australia. These are tier 1 countries and here you're expecting at great conversion rates, but I'd recommend to start only with USA and Canada.
Then there are countries like germany, france and spain.