I think that it's personality related. My DS6 is much more likely that DS8 to engage in imaginative play. DS8 read at an earlier age and enjoyed legos from an early age. He's also naturally more introverted. I think that all of those things play a role (no pun intended). DS8 did not enjoy imaginative play at all until he was at least 6, and even now it's usually led by his little brother or a friend.