From the game's encyclopedia
Health measures how likely a Character is to die by Illnesses and old age.
Old characters have a chance to lose health every year, increasing the older they are.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
Health is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "Health measures how likely a Character is to die by Illnesses and old age." It covers Child Health, Adult Health, Elderly Health and Illness. The game also links it as Healthy. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.