In 7th and 8th grade play grade appropriate rather than age appropriate. Starting with 9th grade play talent appropriate. Play up as far as possible and still get on the field without being overwhelmed. After my kids went through extreme growth bursts I put them in speed and agility training to iron out the coordination.
My kids practiced often away from their teams. We had a net and a tee in the garage. They didn't play fall ball until after 8th grade. But they hit all year even though they were playing other sports.
They started doing weights in 7th grade under the supervision of their personal trainer mother. Being physical late bloomers she was cautious with them the first couple of years.
Neither kid had any additional baseball/softball mechanics training until high school. But they had exceptional coaching plus I played and could instruct them. By high school they were tuning me out so I paid for instruction.
In 8th grade I had them start watching various levels of college ball so they would be able to realistically assess the challenge and ultimately where their talent fit in.