It's a wrong objective anyways. Goes to nowhere.
I am in Bitcoinlandia for many years but never onboarded old grandmas (is wasting time and money). Only young ones with brains.
Making old people to use Bitcoin is a hell of a job, time and resources consuming and 99% end up in give up or worse, they lose control of the wallets and sats are gone.
Their brain is not focused on what really matter, their brain was trained (brainwashed) for decades that somebody else must responsible for their mistakes.
Please when you onboard somebody, remember this meme:
Not sure there's one sentence... you just do it and share it! I can summarize my approach something like: Hey grandma, look, I put some savings here and I'd like you to keep a key safe.
Have a multisig setup with my grandma and another family members 3-5, giving a key to her, help her feel included in this inheritance process. It's taking years for her (ad other fam members too) to understand why I did it. She sometimes spontaneously asks questions, and she knows all money she sometimes gifts to her' grandchild babies goes there. She don't know how to buy, she don't need to... I do it for her.
Making it available without explanation make her curious to learn more. Also provided kids book for her to learn meanwhile she read them out to her grandchild.
Having elder people participate, but not 100% responsible, I found it help spread awareness around small circles (like families).