She thought 10 might have been a later version of the Doctor, and therefor would know her. When she realizes he is not a future incarnation she makes a comment about how young he looks. The irony of course is that 11 was actually a younger looking doctor than 10, a fact not know when SITL was written (Moffet probably assumed the next doctor would look a little older at the time). As far as the 'Who are you' line, 11 still did not know Rivers history or who she really was (Melody Pond), of course know he knows everything about her.
Actually Tennant looking younger to her, I can justify in my mind with how Tennant at that point in time had been healed by Rose and no longer carried he guilt, so, she was talking about the person reflected in his eyes looked young and carefree. Matt Smith's face, indeed is younger looking, but, there is a look in Matt Smith's eyes that does make him look very burdened and world weary.
One Day I hope to be the Man my Cat thinks I am
Where are we going? And why are we in this Handbasket?