Its rare but it has been done, especially if the apritances have a gift for the sort of combat magics required to be a warden. 

and the way I built Regan would mean that she's the sort of warden that would be one of two places: At home base building and healing ( so plenty of time for apprenticing ) or out on the front lines fighting and healing ( Stay home and work on these projects and I swear to god if you sneak into a combat zone after me again I will up and turn your ass into grass. Literally. )