There is not a "better" time to start, its not like a sport. And if you start as a child you cant understand some parts of the theory in music as its too complex...
And yea, jamming with others is sooooo good.
Maybe you don't understand theory, but you can develop really good technique, and then growing you'd learn theory.
I know some guy who follow this way, now they're very good guitarists.
But sure, someone can be very good also if it starts late, it's just that when you're older you've more things to do (a lot), and learn how to play a guitar very well takes time.
True also the opposite, children don't really like playing an instrument and are forced by parents, or get bored after few years.
Points of view I guess.