Messier played for the Cincinnati Stingers of the WHA in 1978-79, they folded in 1979 with the merge, which is why he was in that NHL draft (see Gretzky quote I posted as to why). He played for the Oilers as an 18 year old in 1979-80 (turned 19 in January).
However, it's weird though because 18 year olds, 19 year olds and 20 year olds were picked in that first round...Ramage was 20, Turnbull was 20, Foligno was 20, Gartner was 19, Vaive was 20, Hartsburg was just about to turn 20, Brown was 19, Bourque was 18, Boschman was 19, McCarthy was 19, Ramsey was 18, Reinhart was 19, Sulliman was 19, Propp was 20, McCrimmon was 20, Wells was 20, Sutter was 19, Allison was 20, Mann was 20, Goulet was 19 and Lowe was 20.
Bourque and Ramsey both had December birthdates (Bourque a late one), so perhaps the cutoff was January 1? Gretzky was a January 26th birthday, so him being an 1980 eligible would make sense if the cutoff was indeed January 1 .