There's really no average age that kids stop wetting the bed but usually most kids stop bed wetting by age 7 or 8 years old and some stop wetting the bed as young as 5 years old.
Although sometimes kids continue to wet the bed into their teen years which is normal and nothing to worry about.
My brother didn't stop wetting the bed until he was 17 years of age and he wet the bed from potty training until he turned 17.
He went to the doctor for it and the doctor said that it was normal and he'd eventually outgrow the bed wetting.
During that time he worse some disposable youth diapers at night to keep his bed dry and his clothes dry.
So if your child is wetting the bed it can be a good idea to have them wear some diapers or pull ups until the bed wetting stops.
You can buy youth sized disposable diapers such as attends youth diapers or other sizes of youth diapers and when they outgrow the youth diapers you can go up to adult diapers which are also disposable diapers.