The timeline on the show is famously off. In this episode, Eric turns 18. However, he turned 17 at the beginning of season 1. Since then, multiple years worth of stories have passed, and Eric and Donna's relationship has been said to have lasted over two years since his 17th birthday. At this point in the show, Eric would more likely be turning 20 or 21.
To clarify the time-line error mentioned here previously:
Eric has his 17th birthday in S1E2 (this itself is already a time-line discontinuity because he was stated to be already 17 years old in the pilot episode), in which the opening and closing license plates show 1976. Now in this S6E3 Eric celebrates his 18th birthday, but the license plates show it is 1978. Because we have those license plates to indicate the current year, we don't have to guess how many years have passed. Based on his 17th birthday in 1976, therefor being born in 1959, he should have turned 19 in 1978, not "20 or 21" as the previous submission stated.
Eric was born in either 1959 or 1960, depending on which episode you put more stock in.