> Why is time so special? 

Because it is a part of the condition? How else can you have "older child" in your condition, if time is not important? Stating that first child is a boy is the same as opening the door in Monty Hall problem: it eliminates particular combination.

If you don't know the order of the kids you have three ways to have a situation where two boys are possible: BB, BG, GB. When you know that first one is a boy, you eliminate the GB case and are only left with BB and BG.