Well OP, if it makes you feel any better, I had my first DS at 29 and my mom tries to tell me how to raise him too! Super annoying, I agree!
I deal with it by arming myself with information. Something studied or from a parenting book, such as one of Dr. Sear's books. I purposely leave the book out when she comes over to babysit so that she'll at least read a few pages. Or, if she disagrees with something I give her specific information as to why I am doing it and why I feel it is right. (for example, how can your mom say that breastfeeding so long is wrong when the World Health Organization says to do it for 2 years minimum?) Be very specific and don't make it about your opinion. Make it about following an expert's opinion. She can fight with you, but does she want to try to say that Dr. Sears or the WHO are wrong? And if she does, is that credible?