"Baby boomers and senior citizens" are not a homogenous group, but AARP is a relatively trusted source of information for seniors. Have you considered advertising with "AARP The Magazine?" AARP has about 40 million members.
Something else to keep in mind is that seniors usually have Medicare, approximately 60 million members over 65 years old.

Not sure if that is helpful. If You share more about your model, perhaps I can offer more valuable insight.

Happy planning,

Gregg Michael Kennelly