The people at McDonald's gave my 21-year-old girlfriend a senior discount. It's very possible they just hit the button for your parents and accidentally did it for you. Doesn't mean you look like a senior.
That's exactly what happened, but my parents and I both laughed about it. As an aside, next time I go in there, I'm planning to pay a little more for my regular buffet to offset the money they lost with that one wrongly coded buffet. It was dishonest of us not to go up to the counter and correct it, and I do feel bad for it. It may be a very small amount, and it undoubtedly wasn't the first time it happened in that restaurant, but I don't feel right in having my name or my parents' name attached to that list.
Oh please, considering the meat is probably cheap-grade off cuts... you didn't hurt them.