old people; the elderly
Casual, somewhat blunt term for old people in general. Often used in informal conversation; can sound dismissive depending on context.
This area is full of old people, with few young folks.
It's only natural to give up your seat for the elderly.
grandparents
Only when written 祖父祖母
Specifically refers to one's own grandparents or grandparents in general. The kanji form 祖父祖母 is rare and mainly used in written contexts.
I'm going to visit my grandparents' house during summer vacation.
I received New Year's money from my grandparents.
Usually written in kana.
Katakana spelling used for emphasis or in casual writing.
Kanji spelling for 'old people'; rare and may carry a slightly derogatory nuance.
Kanji spelling for 'grandparents'; rare and restricted to the grandparents sense.
Reduplication of じじ (old man) and ばば (old woman), both of which are informal, somewhat blunt terms. The exact origin is uncertain, but it likely arose as a colloquial compound.