These are elderly people who enjoy good health. The word is, fairly obviously, a blend of well and elderly.
The name for this group of people has come from the health sector where it has been important to do research on elderly people who are not sick to determine what makes them different from the cohort who do suffer from various illnesses.
The term dates back to research conducted in 2007 by the Scripps Research Institute, an American biomedical research institute, on genetic secrets underlying longevity. Their definition of the wellderly was ‘people over the age of 80 who have no serious chronic medical conditions’.
I can see the need to name the opposite of wellderly, that is, the unwellderly, for purposes of comparison. Mostly this research is comparing the genome of the wellderly with that of the unwellderly. The case for unwellderly is clear but evidence of its existence is so far lacking.