McDonald's technically has a senior discount, but it varies from location to location. Each store can determine what discount to give to seniors, if at all.
McDonald's senior discount doesn't apply to the entire order. At the locations that offer it, the discount only applies to certain beverages, typically coffee or small fountain drinks.
If your McDonald's location offers a senior discount, simply ask for it, and they'll take your word for it. Some employees will automatically apply the discount to your order.
If your McDonald's doesn't have a senior discount, don't be too disappointed. The highest reported discount, according to The Senior List, is 33 cents off at a Delaware McDonald's.