First off...there will be no "sick" people on the ship...nothing you can catch from another person. Anyone, of any age, can travel on the ship. Want to jump on in Philadelphia and cruise down to New Orleans for Mardi Gras rather than drive or take a plane? Go for it! Other than some extra doctors and nurses running around, you won't even know you are on a cruise ship providing medical care. You'll get top notch care just like everyone else.
But just in case you need it...you'll have access to care like everyone else, too. Chip a tooth partying too hard during the aforementioned Mardi Gras? The dentist visit is covered as part of your stateroom, no questions asked! You just pay for any medications or supplies you use (which your health insurance may pay for, leaving you with free dental work). We dare you to ask if any other ship, plane, or train can offer such things...that's some safe travel!