There are several reasons why older people have particular difficulty when the temperature is high:
Certain disorders that are more common among older people, such as heart and kidney failure, can interfere with the body’s ability to cool itself. People with high blood pressure are often on low-salt diets, which may prevent them from consuming enough salt to replace the salt they lose in sweat.
Aging also affects thirst. Older people do not get thirsty as readily as younger people. Thus, older people tend to get dehydrated, which in turn means they are less able to sweat in warm surroundings.