In an AMA on reddit, the creators said they had to tell the kids the ending way back in 2006. The rest of the cast just round out. It had to be because of some major plot twist with Ted or the Mother

I read in the AMA with the creators on reddit that even though the main cast wasn't told the ending until very recently, they had to tell the kids all the way back in 2006. I'm not sure exactly what, but i think it points to a major "twist" about either Ted or the mother. Otherwise, why risk a leak or something like…