When the news crew wishes him a happy birthday, the crew reminds Eric that he reminded them it's his birthday. Eric's response was "Well I guess I did." while he is clearly laughing.
In season three, while Eric is working as a weather man and intern at a television station, we learn that it is his eighteenth birthday during a large winter blizzard, but later in the series, it is revealed that he was born on June 24, 1978. It is fairly unlikely that a large blizzard was occurring in June. The episode aired on February 9, 1996, allowing for said blizzard, but not allowing for his birthday.