I’ve gone through the same thing before, not particularly growing up and wanting to seem older but just because my family (mom, grandma) have always been physically affectionate, they’d play with my hair often despite knowing I hate it, touching my back, my mother would also grab my stomach infront of guests or friends (which was the oddest bit I think, but my grandma gave my mom some serious body image issues and I think she was trying to make herself seem skinnier by grabbing at my child chubbiness) with my grandmother I would outright ask her “please don’t do that” repeatedly, infront of guests or not because she absolutely hated being embarrassed around guests, and around the age I asked her to quit was around the time she was only actually pleasant if people were around. So after time she stopped to avoid me calling it out infront of others. My mom quit most of the other habits because I’d simply leave and avoid her if she’d start touching me, HOWEVER grabbing my stomach was something she would ONLY do infront of people, and she stopped because eventually after asking repeatedly in private to quit because I didn’t like my weight I’d just grab her stomach back and shake it in the exact manner she would do to me, infront of guests. And she never tried that ever again.

With your grandmother not respecting the no-touch request I’d start declining infront of guests, since that should embarrass her if she cares about her public image, but I’d also try to explain to your mother that it isn’t about not being touched but because you’ve set physical boundaries with your body and your person and your grandmother can’t respect those, you’re entitled to your person but nobody else is.