quarterquell is the only reasonable answers the interwebs have proffered.
25 years in the past from today
25 years in the past from today
"Quinquennial" per the related link below:
The word century means 100 years FYI: decade means 10 years
The word meaning 1,000 years is millennium.
There is no special word. You could use "quarter century".
A century means 100 years.
25%
A century = 100 years
Elderly.
Ten years
The word is "millennium".