Grindelwald was about two years younger than Albus Dumbledore (DH18). Since Dumbledore was born sometime in 1881 (JKR), Grindelwald would have been born around 1883.
Grindelwald is born
Date circa 1883
Certainty Inferred from canon
Location in Canon DH18: The Life and Lies of Albus Dumbledore
Type of Event Wizarding world