I may never know just why they gave me the ten-percent senior citizen's discount for working on my car, when I was in my early thirties. But I do have two ideas.
See, when I dropped off the car, I left a letter, to say what the car needed -- a proper business-style letter. So, I think it was the letter that got me the discount. But what I don't know is just how.
One idea is that the discount was a reciprocal courtesy -- I addressed them respectfully, so they cut me a break. The "senior citizen" label is irrelevant; this was simply the only discount they could specify in their system.
The other idea, though, is that they thought that anyone who would leave a formal typed letter was necessarily "of a certain age".