It is (supposedly) most likely at XXX, since the client will use old coins/transactions when sending 5 BTC from XXX to YYY.
Is this true? Do older coins get used first prior to newly deposited ones?
I guess it would depend on the wallet you are using. The protocol doesn't specify, and doesn't care. That detail is left for each wallet creator to decide for themselves. In my case, I frequently use coin-control, so I get to pick which output is spent and which isn't.