Acting like a 5 year old

Z turned 3 eleven days before the little boys were born.  After I came home from the hospital, it was like I had an entirely different child.  At first I thought it was because of the babies (and the lack of attention to him, etc), but after a few weeks, I realized, no, he just "turned" 3.  It wasn't pretty.  ;) 
And he has just "turned" 5.  It is pretty!  I feel like there has been a definitive change in some of the things he does.  We have been praising his behavior:  Z, wow!  You are acting like such a five year old, and we are so proud of you! 
For example: 
He sat with the boys the other day and showed them all (very patiently, much more patiently than me!) how to put stickers onto their pictures.  He also has been more willing to play with them.  He is in tune with the different things that each likes, and will get toys for them appropriately (C wants the cars, L loves puzzles, etc). 
His imagination and self playtime has really taken off.  He loves to build with the Legos and he will sit with them literally for over an hour and just create stuff.  He is mumbling to himself too (Ahh!!! You have fire...I will get you...The bad guys got the astronaut...ahhh, venom!!!), and if I stop to listen to him for too long he shuts up and gets all embarrassed.  I always wondered about the kid in Toy Story who played with his toys like that (talking and stuff) and wondered when that sort of imagination actually began.  The answer is apparently age five. 
He has become more helpful (not always, of course), and more able to follow directions.  When I told him to clean up all the Legos because we were going to leave, I walked in a few minutes later and they were all picked up and put away.  He had even picked them up off of the floor.  I have recently been letting him get dressed in his room, by himself, with the stipulation that he had better be done by the time I come get him, or he will be out in the family room with the rest of the boys again.  He has yet to not be dressed. 
That's all I can think of offhand, I know there are more.  But so far, I like age five!