I think it's good to introduce kids to different musical instruments at a young age. Let them pretend to play and explore the sounds each instrument makes. Pretty tough to teach a 5 year old how to play guitar. Kids around 8 to 10 are at the right age to start. My kids both had guitars when they were 5, but I didn't try to teach them much on it, only how to copy me and to pretend to play along with me. I would tune them in open tunings and they would stum open. Then they would learn to stum a rhythm and make up words.
Here's a video example of my youngest daughter jamming away:
Hannah Sings Sweet Emotion