Also its not just restricted to these age groups, players who went undrafted are still eligible. Its not impossible to be selected after going undrafted but its fairly difficult to be drafted. NHL teams prefer to invite the undrafted players to camp rather than selecting them.
But players like Tanner Pearson, Seth Griffith etc in the past years have been selected after going undrafted. Guys like JC Lipon, Cameron Brace, Braclay Goodrow all come to mind of players who may get another chance.