He doesn't have a mind of a seven years old. There are plenty of physiological, and endocrinological, changes that alone would make him quite different compared to a real seven years old. Plus even if he was sheltered and spoiled, he surely wasn't treated like a real seven years old but like a young man who went through something catastrophic and thus a lot would be forgiven. Not to mention he didn't even act like a real seven years old but like a completely spoiled brat his age with no responsibilities and few expectations would behave anyway.

Really, if you had military experience (though I'm sure quite a few of you are bored of me referring to it), you'd see at the age of 18-19, under abnormal circumstances like the military, some guys can still be totally like kids while some are almost like regular adults. So, the absolute age only plays a partial role in defining how mature you ultimately are. If fake-Luke had been raised strictly and properly like a noble's son should have been, he would be like Asch without the bitter edge. Instead, he had parents obviously not suited for nobility.