When your child is no longer comfortable carrying one.
Just take a trip into your local Hot Topic store to see that there are plenty of teenagers who buy shirts and bags with the likes of Hello Kitty and Strawberry Shortcake and other characters. Heck - look how many of US have Disney character shirts and such! LOL!
This is an issue here, as my 5th grader went through issues with this back in 2nd grade. Not a backpack - but her folders, notebooks, etc. She has always loved Pooh Bear, but became embarassed to let anyone know that because many kids thought it was "babyish." I didn't push her, but I continued to reassure her that it's OK to like Winnie the Pooh and if she keeps her eyes and ears open, she will discover that other kids her age still enjoy it right along with her.
She slowly discovered other kids who had "babyish" items or admitted to liking Pooh. Now in 5th grade, she proudly picked out Pooh-related folders for this year.