I hadn't thought about age and altitude problems, but my father tried a move to the Rockies when age about 70, his blood pressure shot way up and a couple years later, that was it. Who knows but maybe there is a study done on this particular age effect. I've always lived around sea level and though I love the mountains, I'm not comfortable at higher elevations.
My suggestions for your bike adventure would be New Zealand with a stop over in Tonga, maybe. There are Enfield rental agencies I believe with quite reasonable prices. and the two main islands there are compact enough to permit great sightseeing with reasonable trips. I have a guy staying here now from the North Island, an American who has lived in NZ for 30 years and was flexible enough to put up with our bridge problem, so I relented and let him stay here. He's out diving today, Sunday and Tuesday, kayaking another day and whale watching also.