That building falling down on the corner of your block? It's decrepit. So is the old man who lives there, if he is weak from age. Decrepit means broken down by hard use.
Decrepit implies that something or someone is a weak, and run down — practically falling apart. It's really not a nice word to use about a person. If you are trying to say the same thing about a person, but more nicely, you might use the word, feeble, which means weak, but not disastrously falling apart.