The problem is that we don't know at the time we diagnose a cancer whether it is overdiagnosed, because we don't know yet when that woman is going to die. Now, you could argue for example—you heard this earlier—that if a woman is in good health and she has a life expectancy of at least seven to 10 years, then we should keep screening her. It's when women are ill with other potentially deadly illnesses that they can stop having mammograms. If they have end-stage heart disease or end-stage renal failure and they're not likely to live 10 more years, then let's not go looking for a cancer that will not be threatening to them before their other illness will kill them.
It's a question of judgment. There's no harm in diagnosis. We don't know and that woman deserves treatment, because she might live another 20 or 30 years. It's more a question of judgment as to when to stop screening.
Many of the screening programs stop at age 74. If a woman wants to continue screening then she needs a requisition from her doctor. There are several provinces that allow women to keep self-referring. That assumes that they're in good health with a reasonable life expectancy.