"Failing health" refers to a gradual decline or worsening of one's physical or mental condition. It means that a person's overall health is deteriorating and they are experiencing various health problems or becoming weaker. Full definition
He was 99 and in failing health, and died on his way to the hospital. (news.berkeley.edu)
For Realtors, this means these boomers will not move by choice but by circumstance, such as failing health or death or financial problems. (remonline.com)
And there are the «old old» who have advanced years plus failing health systems and whose medical and maintenance care needs are complex and individual. (felinefriendlycare.com)