According to Mayo Clinic, vertical ridges in fingernails are normal and have no specific cause; however, deeper ridges may occur as someone ages, possibly due to cell turnover. About.com says that less moisture from natural oils may also lead to more prominent ridges as someone ages.
The appearance of horizontal nail ridges, on the other hand, may signify deeper health problems and warrant a doctor consultation, according to Mayo Clinic. MedlinePlus lists surgery, serious illnesses and arsenic poisoning as some causes of horizontal fingernail lines. It also recommends contacting a doctor if nails turn blue, pale or distorted or if white colors develop under the nails.