There are many possible causes of yellowing, thickened toenails. The two most common are fungus and trauma.
Fungusis much more prevalent among older adults, as well as among people with diabetes and other diseases that compromise immunity.
It's important to get a fungus treated because it can spread to all the toenails and even the fingernails. As infected nails thicken, they become harder to trim and eventually can impair the person's gait, putting him at increased risk for falling. The thickness of the nail can also cause an ulceration (sore) on the nail bed. Or the nail can become deformed from the thickness and then become ingrown on the edges.
Treatments vary from over-the-counter antifungals to prescription oral medications. Topicals are usually only minimally effective but are sometimes used for older adults because stronger oral prescriptions are processed through the liver and require blood work in advance -- and many older adults are already taking a lot of medications. As a result, some cases can't be treated effectively with medication, so we tend to focus on trimming the nails instead.
Trauma can result from a single event (such as dropping something on the nail) or from the nail touching the end of shoes that are too small (repetitive microtrauma). Properly fitted shoes may relieve the latter problem.