Hi Bob,
Costa Rica has two health care systems operating side by side: the public/government-run universal health care for citizens and legal residents called Caja… and the private system.
With Caja there are no age or pre-existing condition exclusions. You’d be able to join once you became a resident no problem. With Caja, you get complete health care (prescriptions, doc visits, surgery… everything) once you pay your monthly fee (7-11% of your income).
In the private system you’d either pay cash (the costs are quite low compared to the U.S.) or use insurance, either from a Costa Rican company like INS or international insurance. I know that INS severely restricts patients at age 70 or above. But I would urge you to check out international insurance options, although I know that the older you are the more difficult it is to get a policy.
Many expats mix and match private and public care.
You can find a lot of info on health care here: https://internationalliving.com/country_clubs/costa-rica/healthcare/
And be sure to let me know if you have any further questions.