While there are some countries that require visitors to obtain travel medical insurance before entering, you are not required to have health insurance in order to qualify for a U.S. visitors visa at this time. It is, however, highly recommended not only for the sake of your own protection, but having it can also be seen as a plus when going in for your visa interview.
Having travel medical insurance will reassure the interviewer that you have coverage for any medical bills that you may incur during your stay in the US. This is especially true for elderly citizens coming to the US, as they have a higher risk for unexpectedly encountering a medical emergency.
So, while travel medical insurance is not required for obtaining a visitors visa, having it will offer you peace of mind during your visit, and can potentially increase your chances for visa approval in the end.
Another commonly asked question is whether or not travel insurance is necessary just to enter the U.S., refer to Do You Need Travel Insurance to Enter the United States?