I initially intended not to tell my grandparents, for the same reason - they're very elderly, in frail health, and old-fashioned. My mother told me (they're her parents) that "it would kill them, maybe literally." My sister apparently got pissed off at my mother and told them FOR me!
I was kind of annoyed at the time, since the decision was taken out of my hands, but they've been fantastic. ๐ As supportive and loving as ever, and they not only get pronouns and name right, my grandmother switched right away to calling me "dear" and my grandfather started hugging instead of a handshake, which is their big "thing" for women vs. men. (Heck, they've been better about pronouns than my parents!)