Being that Saruman is the Maia Curumo, and Treebeard 'only' an Ent, the 'young' can mean three things: young to the lands of Middle-Earth -- Saruman only came at the beginning of the Third Age. Or it could mean that Treebeard knew that Curumo did not take shape until not so long ago, so his body, physical form of Saruman, could be considered young. Or Treebeard simply did not know Saruman was a Maia and assumed he was born sometime late Second or early Third Age only.
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