For most people I would begin with a very light ball not more than 5 or 6lbs.
If you are dealing with children probably much lighter.
I was a competive gymnasts up to age 13, a lover of water and jumped my height 171 by age 14 and was competing nationally before highschool.
When I met Charlie at age 21 I was using the lightest med ball for 2 full training years before touching a weight ( other than med ball) and I believe it was 2 or 3k. I’d have to check my journals.
Woman will be very different than men ( very surprising I know) when it comes to strength.
Look for quality of exercises not quantity of weight.