Yes, if you and the eleven-year-old aren't intending to do anything sexual and you and her parents don't mind. My greatest advise is to consult with both sets of parents. Being friends with an eleven-year-old girl may not be a good idea if you are a heterosexual fourteen-year-old male. There's no telling what they might do together! The only real rule here is that everyone needs to understand that they're peers in everything but age. If she's willing to let you call her your friend then go for it. You might want to check in with your parents first though since they'll be the ones responsible for enforcing any rules you make.