There are changes in the senses of taste and smell as we age, and that affects the flavor of food.
These changes are gradual but, by the time people are in their 80's and 90's, food may have little appeal for many people because it may have too little flavor and taste. Food may need to be more highly seasoned in order to taste better.
And, in that age range, some people are on salt-restricted diets, due to hypertension or kidney problems, and that may diminish the flavor and appeal of food even more.
Some older people also eat fat-restricted diets, when there is really no longer a need to do that, and a person in that age range who is too thin may need higher fat foods in order to boost their calorie intake, and higher fat foods may also have more taste appeal.
Beside sensory changes, many older people often suffer from depression, even a low level chronic depression, and that does affect appetite.
Being very thin is not healthier for the elderly, quite the opposite. So, it's important to do everything possible to make the taste and flavor and texture of food more appealing in order to insure that there is adequate calorie intake, and smaller more frequent meals may help with that. The liquid supplements, drinks like Ensure or Boost, taken as a snack rather than a meal replacement, may also help with increasing calories.