Geriatric often describes something related to the medical care of elderly people, like a geriatric hospital. When it’s used outside of a health care context, it takes on a sense of “feeble” or “decrepit.” It can be humorous if you use it about someone or something that's not actually old, like a group of middle-aged people calling their weekly cocktail gatherings the “geriatric club.” But if you call your grandparents’ golf buddies the “geriatric club,” they’d probably be offended.