I would certainly call "considering" a preposition in your example. The structural difference between "considering" and "concerning" is that the latter takes an NP complement, whereas "considering" can also take a content clause, as can be seen in your examples. Subordinators ("that", "for", "to", "whether" and "if") are merely grammatical markers of subordination, whereas preps function as 'heads' of the expressions they introduce.
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I would certainly call "considering" a preposition in your example.
The structural difference between "considering" and "concerning" is that the latter takes an NP complement, whereas "considering" can also take a content clause, as can be seen in your examples.
Subordinators ("that", "for", "to", "whether" and "if") are merely grammatical markers of subordination, wherea
Considering can act both as a preposition and conjunction. The example given here is a conjunction. In the sentence " She is very active, considering her age", considering is a preposition.
http://www.oxfordlearnersdictionaries.com/definition/english/considering