I have an online platform designed for use by baby booms & senior citizens. However since (I think?) they don't typically spend as much time online as the younger generations, then how or where can I go to effectively market to this audience online? Offline marketing suggestions are less relevant at this time, but always valued/appreciated!
You may be surprised to find that Baby Boomers and Senior Citizens are fairly active in online communities and social media because this is how they stay connected to family and friends. I wouldn't ignore it entirely.
Two things I would do: First, If your online business lends itself to creating content of some kind that is educational, look to content marketing. Create a blog and write content that answers questions your target audience wants answered. Share that content on social media. Facebook and YouTube are great places. You've probably heard that younger people are turning away from places like Facebook because that's where their parents and grandparents hang out online. Well, there you go. That's where your audience is. Become an authority in your space. Your audience wants to know they can trust you if they decide to part with their money and spend it with you.
Second, and this is for offline marketing because it shouldn't be overlooked, look to your local area and find associations or organizations that cater to your target audience. Many of these organizations host meetings or group outings. Could you perhaps take your same educational content and present it to these people in person? Start small and you'll be surprised that word will get around about your product or service if you do a good job presenting it and demonstrating its value.
I know my answer is somewhat ambiguous because I don't know much about what you are selling or your online platform, but look to content marketing. If you can skillfully craft your pitch in an educational format, your audience will likely respond accordingly.