This may seem like trolling, but I'm not: direct mail is extremely effective for Americans 55+. However, cost per piece is pretty high and response rate is still single digits tops, so you need a product that warrants a customer acquisition cost of dollars, if not tens of dollars. If you have that, this channel will scale.
In contrast, targeted Facebook ads probably have better unit economics, but I'm skeptical they will scale to cover most of this demographic. However, if you're just trying to test something, that seems like a pretty easy, low-investment way to reach people... Every demographic is represented on Facebook and it's pretty straightforward to target ads, especially if you don't care how large it could scale.
Google consumer surveys are pretty easy to use if you're just trying to get opinions, though - in contrast to being a Facebook user - it does seem like conditioning on having Google Surveys installed on your smartphone might not provide a representative sample of opinions for retirees.