The Garrett ACE 400 would be a good metal detector for older kids as it’s designed for adults. It weighs 2.9 lbs, so it’s lightweight and can be used without strain for long periods of time. The shaft length is also adjustable from 40” to 53”, so it can be retracted down somewhat for shorter users.
This is not a kid’s metal detector, but older users from 12+ can be taught how to use it and can use it comfortably.