MIAMI — With all the challenges and complications as one of the most powerful hurricanes ever recorded bears down on South Florida, there is also this: One in five residents here is over the age of 65, a percentage greater than any other state.

In the three counties that appear most likely to bear the brunt of Hurricane Irma — Miami-Dade, Broward and Palm Beach — roughly a half-million residents are over the age of 75.

They range from active residents living in lush retirement communities to physically and mentally impaired people living in assisted living units or nursing homes, but as previous storms like Katrina, Sandy and Harvey have demonstrated, they are among the most at risk. The sheer numbers and vexing variety of the elderly population poses enormous problems as officials must assess risk and allocate resources, both before and after the storm.

Because so many older people move to Florida later in life, they often do not have the family and neighborhood connections that would provide support in an emergency.

“We call this group elder orphans,” said Jeff Johnson, the state director for the AARP. “Many of them are probably in a condition that is normally manageable, but if left without power or stranded for days, away from support and care, when the state doesn’t know who they are or what they might need, that is the worry.”

Mr. Johnson said that state and local officials had worked hard not to repeat the mistakes of storms past, and advocates for the elderly “have pretty high hopes that the state and local authorities are attentive to people in those sorts of institutional settings.”

Since the end of the Second World War, retirees have been lured to Florida’s sunny shores by affordable housing, effective marketing and an idea of the good life. They first settled in and around Miami and St. Petersburg but the 1960s saw the rise of gated communities across the state.

During normal times, many of them would be fine on their own, perhaps taking offense at any suggestion otherwise. After Irma makes landfall, times will likely be anything but normal.

The concerns range from caring for the sick and infirm to persuading people reluctant to leave their homes that it is wise to do so.

“You saw the picture of the lady knitting with the water up to her waist,’’ said Mike Graham, 71, referring to the now infamous photo taken at the La Vita Bella assisted-living center in Dickinson, Tex., in the wake of Hurricane Harvey.

“It is sort of like: We have been through it and this is what it is,” Mr. Graham said.

Mr. Graham was living in Miami during Hurricane Andrew in 1992 and rode that storm out. Since then, he has moved to the beach and lives in a high rise. He is leaving but he is not going far, heading downtown to his brother’s house.

“It survived Andrew,” he said.

A retired general surgeon, he is not blasé about the risks, but realistic about the options.

His mother, he said, is 94 years old and lives in an assisted living facility in Coral Gables.

Since he did not expect flooding to be a problem, there was not much to be done other than to make sure she had what she needed.