here's mine: when you start labeling gifts to your children from the pets...
(instead of mom or dad, you start saying they are from simon or rosie (our cats) or alfred (our hamster) or chubbie (our bunny) or...)
please - feel free to add yours :)
also i thought when they start opening presents you gave them that even you are surprised about. that is either over-spending or short-term memory loss.