According to the book Encyclopedia of Nails (Thomas Learning 2006), fingernails can be soft for a variety of reasons.
Medically referred to as hapalonychia, a soft nails can be the result of too much water being retained in the nail. Soft nails are linked with a few medical conditions such as eczema and dermatitis. Soft nails caused by conditions of skin inflammation generally start at the base of the nail bed and work their way up. And frankly, some people just have naturally soft nails.
According to Dr. Steven L Schwartz, anatomy and physiology instructor at Indian River State College, soft fingernails can be caused by low stomach acid, which 60% of North Americans experience by age 60 and 80% of North Americans experience by age 80.