Is it normal for a 9 year old being that serious!

Hi everyone. The youngest of my two little brothers is 9 years old, and from what i can see he's way too serious. He hardly ever laughs or smiles about anything, and apparently quite enough of them are fake smiles. He doesn't get enthusiastic with many things either, almost any at all. He doesn't talk much and basicly tries to avoid almost every conversation. He always was a serious and introvert boy but i think now he has become even more. Kids his age are most of the time silly and get amazed by every little thing. And i know that from first hand since my other brother, the 12 year old one, is the silliest, super energetic and most anoying creature ever. He never misses the oportunity to fool around and do the dumbest thing he can think of. I know that brothers can bee very different but the thing is, i don't feel he's acting like his age. And most of his friends are more or less childish but he doesn't seem to act himself with them either. Also he asks many questions about death. A few days ago he asked me what he'd do if i died and he was left all alone. I haven't been able to figure why a little child would think of such a thing yet. Also his teacher said he's not very interactive and some of my friends has noticed that too. So should all these been taken into consideration, or it's just me and he's just a little differnt from his coeval kids?