Parkinson's disease is similar to attention-deficit hyperactivity disorder (ADHD) in the sense that both are thought to stem from dopamine deficiency, though the affected areas may differ. Executive functioning is typically associated with the frontal lobes, the prefrontal cortex, the anterior cingulate cortex, and parts of the basal ganglia (including the caudate nucleus). Indeed, those suffering from Parkinson's have critically low levels of dopamine in the caudate nucleus, and thus patients have shown to improve specific cognitive functioning abilities with dopaminergic medication, such as those used to treat ADHD. This is likely due to the rewiring of the dorsolateral prefrontal cortex and the posterior parietal cortex to the dorsal caudate nucleus after medication was administered. 
However, most medications used to treat Parkinson's typically try to target motor symptoms. This is because PD is most associated with dopamine depletion in the substania nigra, as well as progressive effects on the nigrostriatal dopaminergic pathway.  On the other hand, ADHD medications (such as Adderall, Vyvanse, and Ritalin) target the mesolimbic and mesocortical pathways, allowing them to influence cognition and affect.  Thus, there is evidence the cognitive deficits observed in PD may be treatable with psychostimulant medications commonly used to treat ADHD. Overall, there have been comparatively few attempts to treat cognitive deficits in those with PD , though there have been modest effects with treatment of Modafinil for PD-related daytime sleepiness.  Though Modafinil is a wakefulness agent, it is also used recreationally as a mild cognitive enhancer (or 'Nootropic'), and is said to boost alertness and cognitive function in otherwise high-functioning adults.
As every person is different, 15 years of Parkinson's will manifest itself differently in every individual. Tremor is usually the first symptom of early-stage PD. In the next stage, it spreads and causes the person to have slow movement, stiff muscles, and poor motor coordination. Poor posture, difficulties with writing, and trouble balancing may develop. After a certain number of years (relative to the individual), the person may be confined to a wheelchair or a bed. Dementia may occur in up to one-third of these individuals. (See: WebMD) As it stands, there is no medical 'cure', but the medications can delay the end result.
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 Brennan, L. (2013). Dopaminergic Therapy in Parkinson's Disease: Psychiatric Symptoms, Cognition, and Everyday Functioning (Doctoral dissertation, Drexel University).
 Oades, R. D., Sadile, A. G., Sagvolden, T., Viggiano, D., Zuddas, A., Devoto, P., ... & Russell, V. A. (2005). The control of responsiveness in ADHD by catecholamines: evidence for dopaminergic, noradrenergic and interactive roles. Developmental science, 8(2), 122-131.
 Adler, C. H., Caviness, J. N., Hentz, J. G., Lind, M., & Tiede, J. (2003). Randomized trial of modafinil for treating subjective daytime sleepiness in patients with Parkinson's disease. Movement Disorders, 18(3), 287-293.