Why is the creation date of the bitcointalk account an important factor?
I don't think it's important, it's probably an arbitrary choice.
Trust settings uses the same order. From a programmer's perspective it makes sense to just sort the (user) numbers.
So if your account is older than mine but we send the same amount of merits for the same post, your name would appear before before mine every time?
Correct 
I stand corrected!
By "oldest account first", LoyceV tried to mean the account which merited the post first followed by the subsequent accounts meriting the same amount of merits.
No, that's not what I meant. It's also not how it's sorted.