There are limited if any medical regulations for politics. With the exception of the executive branch where one person is supposed to be in control, in legislative branches no single senator or rep is required for it to function. A dead senator doesnt stop government function and it is up to the voters and state laws for placement or replacement. The US is a gerontocracy for many reasons, a big one is that seniority matters for political power, e.g. committee appointments, name recognition, etc. Multiple important politicians are in their 70s-90s in part from inertia and in part because of the power of the baby boomer voting bloc.