Hagrid’s birthday is December 6 (JKR), so he turns 12 a couple of months after starting Hogwarts.
Meanwhile, Tom Riddle is starting his third year.
The year is based on Hagrid's birthdate of December 6, 1928.
Rubeus Hagrid starts at Hogwarts
Date September 1st, 1940
Certainty Inferred from canon
Type of Event Wizarding world