Interesting thread..
my DD, 10yo and 70 lbs, uses a 31/19...
She went down to a 30/17 for a few practices, but just wasn't hitting the ball as solid...
my 10yo is about 90 lbs or so and swings a 30/20 without issue. She tried a 30/21 RT but couldn't swing it vert well but that might have been the bat design and not the weight.
Each DD is diff. Mine has used a 33/24 RT since 11 years old. Swings it like a feather. 5'6 140 lbs, but she is hard as a rock. We moved up to the 34/25 RT this year at 14, still like a feather. She doesn't care for hand loaded or balanced bats. But I will agree that her bat is way too heavy for most. Some of her team mates have tried to use it, they have to drag it to the plate. It's pretty funny watching them.
I agree with GD on this one. It's really an individual thing. My 12yo has been using a 32/22 Voodoo very effectively for 2 years now. She's not a big kid I think she's barely over 100 lbs and skinny, but she totes hay bales, 50lb feed bags and water buckets all over the farm so she's quite a bit stronger than a lot of kids her size. It's kind of funny when a bigger kid tries to use her bat and can barely swing it.
My kid (10U) was swinging her sister's 31/22 RT at our first tournament. Pitching was very slow and she was hitting the snot out of the ball. I picked up a 30/21 RT for her and it's perfect. The sizing chart is a great place to start, then you have to pay attention to see how well they swing a certain bat. Fortunately, I have several bats of various sizes/weights.
Parents often want to know what size bat they should be swinging. I tell them to wait and I'll let them know. I can give them a pretty good ballpark number, but it's not a one size fits all. I hear the Anderson NanoTech is weighted differently and you can bump up to the next size/length and it feels the same as the lower size/length composite.
I go on the light side until they can handle the bat well enough to move to the heavier bat. You don't want bad mechanics because of too heavy of a bat. But you don't want too light of a bat, either. It's something I analyze and decide with each kid. I'll switch bats up on them during tournaments by a bigger drop or smaller drop to see if they're still swinging properly.
At least that's how I do it. Others may have a different approach. But normally, the biggest bat they can effectively handle is optimum.