Why is it considered dangerous to give your name out online?

[Gabrielle Santiago]

I understand this is a basic rule to internet safety. Along with not giving out your name, telephone, address, etc., but say you don't give your name at all. Or your location. Would giving out your age still be considered dangerous? or say maybe you do give your name, but only your first? And if you were to give out your location, what if you just put the country, and not the specifics like a city or state?
Sorry the main title was supposed to be why is it dangerous to give your AGE out online