Do you think an 11yr old is too old for spanking if she is defiant, rude and disrespectful to her mom?

No... She lives under your roof meaning your rules. If she is disrespectful and rude a few spanks on the bum is what she needs. Punishment is necessary especially at the age of 11 where a girl has to start make choices for herself. Spanking her will help her realize that she is just a kid and needs to follow rules whether she likes it or not!! Make sure if you spank her its for a good reason like cussing or tantrums or skipping school, back chatting is also a good reason but don't spank her for little things like a messy room or forgot to do her homework (Only if you told her to do it and she didn't). I will say the age to stop spanking is in her last year of high school (18) or when she starts behaving I hope I've helped =)