Cats and Puberty

I left the title open for general discussion, but I do have a specific question.
My older cat, who is already spayed, hit her puberty at about 6-7 months (by which I mean she started going in heat) which I understood was the common time.
But my male cat, who is not yet neutered due to some circumstances, hasn't shown any signs of puberty (like spraying, aggressiveness..) even though he's over 1.5 years old! Is this normal for an indoor male cat, should I be worried or is it just a strange case?