To me the age would be how I would feel about them listening to music. If you have a three-year-old then rap music is not wise. Teens can hear that music, but just make sure that they are respectful to you.
Well, rap music wasn't something my kids had access too when they were three and I wouldn't just offer it to them lol. At three, they're still listening to cute childish things and happy music. One of them is a teen now and the other about to be and both are extremely trustworthy, honest, respectful kids. They were raised with respect all around and that's what I got in return. They can listen to whatever they want, because they know they're just words and it's the music itself they're listening to. They have respect for women/property/other people in general/the law etc etc etc so no song is going to override that and make them rebel. I raised them in a way that they have nothing to rebel against