"Granny slang is the colloquial parlance of the elderly. Its nickname is, itself, slang, which is perhaps why it has taken on a superfluous gendered personification. It's not necessarily effeminate; granddads are just as likely to use granny slang; it denotes age, not gender. Admittedly, 'granny and granddad slang' doesn't have quite the same ring to it."
It's from across the pond, but people get old there too.