How and when are most children who are mentally disabled potty-trained?

I've worked with mentally handicapped kids and adults for several years. Some of them never potty train. It depends on their disability. Others may potty train 'late' by our standards (5-8 years old or so), but this delay is normal for the child. The child's doctor may be able to refer the family to a behavior modification specialist who can provide potty training advice and methods. The important thing is to keep positive and just hang in there.