I would argue it's probably OK, and maybe even ideal, to hold off until U9 or U10. That gives your kid some time to play some other sports at the rec/church league level for a while, and puts off the grind of travel for a bit. Better to have him/her chomping at the bit for a chance to play travel than already feeling burned out going into high school.
But as others have rightly noted--every kid is different. Full disclosure--I didn't follow my own advice with my kid (started him much earlier), and he stuck with the sport and still plays as an adult. But I kinda think that was just more luck than anything else; a different kid could have easily burned out in his early teens. So what do I know?
The coach who posted above is probably worth listening to, I'm just a retired soccer parent.
