I did want to point out that my own grandparents are 75-ish, and use gmail just fine, and have a blog that they update regularly.
So I'm not sure if this market will be increasing, or decreasing. Over a long enough time frame, I think it probably will be decreasing, as more tech-savvy users get older.
Also, my father's generation is more used to paying for things of value compared to my generation. It could be a good market.
The market for older people who need a different mail client because they don't know how to use email is probably on the decline. But, the market for older people who need a different mail client because their eyes are going bad or they can't type well anymore is probably on the rise.
One other comment is that PawPawMail sounds like you are marketing to grandfathers. In my limited experience, the grandmothers (and aunts, moms, daughters) send far more email.