That's definitely a tricky audience to get in front of online, since those generations aren't on social media nearly as much. Here are a few recommendations:

1) Target a younger audience (and/or baby boomers, who have a stronger online presence than seniors). Figure out how to market your service to younger folks by tailoring your messaging around them wanting to preserve memories of their grandparents/parents/etc. I'd also push hard on the gifting aspect here.

2) Run a test campaign with Facebook ads and use the age targeting setting to display your ads to baby boomers. (This is also a good segmentation technique for millennials if you choose to go with the "grandparents" messaging.)

3) Find other organizations with a similar audience and think of interesting ways to partner with them. What other sites do members of your target audience visit? Are there other apps or services that would complement your site nicely? Contact those folks and strike up conversation, learn more about their businesses, and suggest some potential ways you could help each other gain more customers.

4) Leverage your positive reviews and ask your happy customers to tell their friends about you! Given the demographic you're after, this will more than likely be via word of mouth (not social sharing or email forwarding), but WOM is the most enticing "referral channel" for this group anyway.

Best of luck, and feel free to let me know if you'd like to chat further & come up with some more ideas together.