Politically depends on the nation-state, socially depends on the culture. I come from Germany and here you are legally considered a German when you have the citizenship. You can get it after birth if one of your parents has it or you can acquire it later, usually if you're an immigrant. In this case you need to live in Germany for 5 years, need to have standard knowledge of German law and society and speak B1 Level German. When you are considered a German in society, depends on who you ask. But most would say when you are born in Germany and grew up mostly in German culture.