Started my daughter on carrying a pack at about 9. It’s mostly when hiking since we don’t get to do much meat packing out here in TX. Picked up a new pack from Gregory for her this year that fits her well with a 30L capacity
I started my kids out with camel pack's at 8yrs old , there small and and comfortable and they love drinking water out of them and can carry some snacks for the day as well . I think the key is not giving them a pack that's to big or awkward so they can enjoy there time out there , and if they happen to get one you can still get it tied on there so they can help with the pack out.