Why are seniors such terrible tippers?

I am used to getting 15% of the bill (minimum) for my service as a waitress. But when it comes to seniors you're lucky if you get at least 10% of the bill. I dont understand it, breakfast time they get free coffee and breakfast is $4.25 so i don't believe they cant spare 2 bucks for a tip. I fill thier coffee's at least 7 times and when i walk back to the table to clear it find 50 cents. What the heck is that about!? i even make an extra effort with seniors to try and increase my tip, but it doesn't help.