C.P.
Musical talent runs in my family. I've been a vocalist since I can remember, but did not start any formal training until age 8. Even then, it was tough to focus.
My son is an excellent guitarist. He also started at age 8 and is now 10. He has an innate ability that was recognized at about age 6, but I waited because I wanted him to be exposed to different types of music and performance.
I think it really depends upon the child...but I do think that age 5 is really too early to know for certain where to start. Take him to listen to music and see what have gravitates towards. My oldest has always had an affinity for guitar, but my youngest can sing like a bird and keep a beat through an entire set of songs (drums! bass!). Watch his hands and feet. His fingers. You'll get an idea.