What age did your kids switch from -10 to -8 to -5

My kid (10U) has swung a 28 -10 for the last two years. We do a bunch of backyard soft toss BP with a 30 -8 wood bat and heavy balls that he enjoys trying to hit over the fence. He's about 4'10 and only about 70(ish) lbs but he shows pretty good control with it. So was thinking about buying a 29 drop 8 for the remainder of this year and next. He doesn't play at the highest level so some of the pitchers throw meatballs that don't require the quickest swing speed to catch up to.

When did you know your kid was ready to go to drop 8 or 5? Is there an age/size that is generally standard?