Going with the 25 and under with 65 or less games played at the NHL level which actually works pretty closely with my own rule of thumb of "anyone not too old that hasn't been penciled in".
Therefore Granlund, Ortio and Ferland are still prospects.
Monahan and Gaudreau are young but clearly established and no longer prospects. Bennett may be penciled in but he's only played 12 NHL games and is a prospect.