Our pediatrician told me that you subtract the child's age from 10. That number multiplied by 10 is the percentage of children in that age group that stll wet the bed.
At age 7, for example, 30% still wet the bed.
Heavy sleepers have the biggest problem. It's best to let your child feel wet when they get older rather than put them in overnight diapers or underwear. It trains them to wake up and use the bathroom. The exception would be for an overnight away from home.