If you're going to be camping you want to make reservations ahead of time because at Oscar Scherer the campsites are always full. When I make my reservation I just put that I'm a senior and it automatically deducts the discount and then I just show my driver's license when I check in.
When you come to the park you will stop into the ranger station and they handle all that simply by presenting your ID. If you meet the required age threshold they will apply the discount to a pass or camping sites. I do not believe there is a discount for day use admission.
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