You tend to use fragile to describe things, whilst 'frail' is used to describe people.

"Fragile" means that it could easily break, either literally (china, pottery) or figuratively (a "fragile" person gets their feelings hurt easily). "Frail" means "sickly" or "weak" and nearly always refers to a person.

"Frail" is usually a result of someone becoming very old or very ill.