You're a lot kinder than I. I would have shoved the sub back at him asking for a refund, then given him (politely) a piece of my mind. Later I would have called to speak to his manager and ask if it's his policy to treat all seniors like dirt.
Although rarely discussed discrimination against seniors is universal. In any other culture than the Western, elders are held in high esteem and it's considered an honor to take care of one's parents in their twilight years. Elders are thought of as wise, and their advice is highly sought. Grandchildren are taught to exalt their grandparents.
But, hey, we live in the greatest country on earth so I'll take my knocks as I get 'em knowing I did my part to make this country a better place, as much of our generation did....