FWIW, my oldest started travel at second year 14U and is now pitching in college. Didn't start pitching at all until 13. That wasn't by design, I knew nothing about the existence of travel ball. My younger kid started at 10U which to me is plenty early.
A better question to me would be what age should they start playing the game at all. My vote there would be 8. Few kids younger than that have enough coordination, strength, and most importantly attention span to play. I really think that is why you see a lot of kids quit both softball and baseball at a young age. They get bored because nobody can play it with enough skill to make it interesting and frustrated that they can't do anything right. I doubt parents are the only people at a t-ball game cringing at the abomination it is. On top of that, many develop bad habits at the very young ages to compensate for the lack of strength and coordination. Thus you end up with some horrendous looking swings, poor throwing mechanics, and wonderful memories of getting hit in the nose by ground balls because they can't control the glove. Think about t-ball and the goal of the hitter and how best to accomplish that goal. They want to pull everything to the left side (because they are ALWAYS safe) and swing up at the ball so it goes in the air as far as they can because it looks pretty and nobody can catch it anyway. Not at all the same goals you have when you are doing tee work for skill enhancement. Why is a kid playing the pitcher spot? They are one of the early learners who can actually catch something and throw the ball. The list goes on and on. Far more dirt piles are made and dandelions murdered than anything else at that level. So why not do everyone a favor and just skip it. They can catch up pretty quick when they have the coordination to actually do what they are supposed to do.
While i loathe sokker, the only time it is the least bit entertaining to me is when they are little. Small field and lots of action. Any kid at 5 can run around and kick a ball. Every kid can make a contribution because the ball will bounce their way and they get to kick it. The older they get in that game and the more skilled they become, the worse the game gets from my standpoint. Play sokker at 5, graduate to a better game at 8.
Please keep things orderly when you show up at my door with pitchforks and torches. Yes, I know SOME kids can play softball/baseball pretty well at 5 or 6, MOST can't.