My kids are 100% vegetarian from birth and I can't say we have observed any issue whatsoever. We do ensure they eat a very varied diet. We do get their bloods checked from time to time and nothing notable there either.

Of course it's possible they would have been gigantic, athletic super-geniuses if we'd fed them meat. I can't disprove that, except to say I ate meat as a kid and I'm decidedly average.