What causes prominent veins on the forearm?

Exercise: Exercise tends to make veins more prominent.

Yes.: Veins return blood back to the heart and can carry a large volume of blood. Depending on many factors including levels of hydration, genetics, outside temperature and even physical activity, the size of the veins will vary. Another important function of the veins is to help control body temperature and this too will affect vein size. Also with aging, veins enlarge as tissue volume decreases.