Diet & home care. Eating onions and garlic and nor brushing well are a majow cause. Sometimes dry mouth can cause it.
Check dentures. Very after dentures may be the cause .Also adequate fluid intake & oral hygiene will help.
Not because of age. Bad breath is not due to people's age, most bad breath is due accumulation of bacteria in between the teeth, gums and tongue, old people may have some difficulty brushing and flossing properly, thus a high level of bacteria remains in their mouth that ferment food particles and produce foul odor gases.