You know what I found shut my mum up, until such a time as she could see I was doing the right thing by DS? Every time she said "are you going to be feeding him at school?", or "are you going to be feeding him when he's 12" kind of comments, I'd shoot back "what if I do?". She backed off and the next time she mentioned anything when I breastfed, it was only support!
In fact, after the negativity and when the positivity came from her, she extended it to look at us and sigh "I wish I'd fed you for longer, I don't think I tried enough". I was fed for 6 months, and by 'tried enough', she means to go against the prevailing custom in the 70s to use formula, in the mistaken belief that Western ways must be better. My sister, who is younger, was fed for 3 months, before being fed formula and developing eczema that she still has.
You'll never be called into question by a mother who breastfed her children full term, or has reconciled her breastfeeding experiences with herself! It's the ones who see your breastfeeding practices as a criticism of what they did, who just need to be shown that you're only doing what you believe to be right for your child, who take your continued breastfeeding as an affront to their expectations.
Weird, coming from my mum, given my grandmother was a full term breastfeeder - but I suppose my grandmother represented to my mum all that she wanted to leave behind when she left for Australia, thinking everything Australian was better (she acknowledges now that it's not true - she is glad I've reclaimed the kind of birthing my grandmother did and ignored 'doctors orders'!).
So, I took some time to examine what was going on for my mum when she took exception to how I was parenting, and approached her more sensitively, knowing she wouldn't even realise where her own reactions were coming from.
You could give her a smart arse response to get her to back off, or you could tread lightly, patiently and gain her respect I would never have thought it possible in my own situation with my mother, but whaddya know?? I did it!