"a third answer being get good coaching (proper training is a big problem at the younger levels)."
I had a bet that I unfortunately won with two of my 13 year old all stars. Their throwing mechanics were so absolutely ugly that I bet them they'd injure their arms by their senior year of HS. But these two kids had the strongest arms around, so no one ever wanted to touch their mechanics, and because I was the first and only one to bring it up, at 13 years old, they weren't too interested in changing.
A big problem I see as a current Jr, High, select and former Rec. League coach is kids swing bats that are to heavy for them. Parents make the mistake of when a kid moves from coach pitch to player pitch that they need to get a longer/heavier bat and the kid cannot get it through the zone.
There are some very good answers here. As a high school coach I see all of these problems with my younger hitters. Arm barring, high elbows, bad pitch selection, not getting the hips through the zone properly, and kids wanting to swing the biggest bat possible. I think bad coaching at an early age (yes you daddy) and kids swinging the heavy bats creates many of the other issues. Our best 9th grade hitter was swigning a 31 inch wood bat this summer and choking up. And hitting piss rods.
One of our varsity guys was swinging a 34 wood. I suggested he hit with a 33 and he hit 2 doubles his next 2 AB's
" kids swinging the heavy bats"
"kids swing bats that are to heavy for them"
I play in a 35+ MSBL and I swing a 32" M110. I can hit the ball well over 300 feet with it. I'm 6' 180 lbs. When facing a particularly slow pitcher, I might go to a 33" of the same thing. I have no idea why anyone smaller than me would swing anything bigger than my bat. I see it all the time too. What makes it even worse is the parents go to their local Modell's or Dick's or Sports Authority, let the kid pick out the metal bat, argue for a little bit that the bat is too big for them, then remind the kid that it's the only bat they'll get this year and the proceed to pay $300 for the new 33", 30 ounce bat for their 13 year old. After a few games and strikeouts, they then see their kid using someone else's 30/27 and hitting lasers.