Children and bathrooms

A friend and I were talking about kids and the toilet. They brought up the fact their kid doesn't always flush so as punishment dad did his business and then made the child come in and flush. I said I think its a kid thing. My daughter does it too no matter how much I remind her. So how old did your child remember to flush and not have to be reminded? I think maybe by 8/9 we won't have to discuss this .. Opinions