When I owned my business, I was on the same group health plan that my employees were on, except using the VA for treatment of wounds sustained in battle.
When I turned 65, being as it is ILLEGAL to buy private health insurance, one doesn't have much choice, as it also ILLEGAL for a private insurer to sell policies to those 65 and over, with the exception of Medicare advantage, which Obamacare will eliminate once it is fully in affect.
It's not illegal to buy private health insurance when you're over age 65. Some people actually don't qualify for Medicare, and have to pay a premium to get Medicare Part A (which is free to most everyone who has paid into the system or has a spouse that has). They
could buy private health insurance if they could afford it. Most of them simply can't afford it, so they pay for Medicare instead. (That was the whole point of Medicare - old folks can't afford to take care of themselves because they aren't working anymore.)
Also, people who are still employed at age 65, and are covered under their employer's insurance plan, can keep that IF THE EMPLOYER'S PRIVATE INSURANCE ALLOWS. A lot of employer sponsored insurance plans simply refuse to be the primary coverage for people over age 65 because they are in a higher risk pool and are more expensive, so they opt to be the insured secondary insurer and force the employee to take Medicare as their primary insurance.
It's not a question of legalities, it's based on what private insurers are willing to provide. And most private insurers just don't want to insure old people because they are expensive and will get rapidly more expensive as they age.