As a father of 5 maybe 6 boys (in May), I give this one a lot of thought.
Each kid is a little different of competing. Practicing and learning fundamental technique can happen at any age, but if the kid ain't feelin' it don't try to make them concentrate, it will just make you mad.
Competition with other opponents, start slow. Smaller is better than bigger. Bigger tournaments require many wins in order to place and awards are good.
As far as training the way most of us consider it, only a few wrestlers can handle hardcore stuff before about 6th grade. After that if they are going to wrestlers they will. I have a Pee Wee, Bantam, and Novice. The youngest hasn't wrestled yet, Bantam wants to be Indiana Jones and I took Novice to FS/GR State last year. He won a couple and made great progress but he burned out too and he isn't competing this year. At 10 years I am not giving up.
I used to think kids that started early had an advantage. After over 20 years of watching kids divisions at local tournaments, many of the kids wrestling young are not the same ones in high school.
I would rather my kids wrestle later rather than earlier.
eryc