2 Answers

Old enough to be ready, willing, and able to support the potential consequences. 
Just a little thought to consider.  If she is under 18 and you have sex with her, you can be arrested for statuatory rape, regardless of your age.  If she is over 18 and you are under 18, she can be arrested for the same.  (I'm getting the feeling you are 15, just from your nickname)
Get your own sleeping bag.  Anyone inviting you to share her sleeping bag at age 15 is not doing you any favors.

LOL !!!!!!!!!!!!!!!!!!!!!! For what purpose ????????? Question should be : How old are you when you should stop shareing a sleeping bag with a girl . Very funny !!!!!! If you have to ask this then dont be shareing any sleeping bags with girls for a while longer