Okay, so it is not just insulting to other knitters when you tag your knitting with “granny” or “grandma”, it is also insulting to grandmothers.
Let me explain why: Your grandmother was not born a grandmother. She did not suddenly acquire the “knitting skills” achievement when she leveled up to grandmother. She probably learned to knit when she was a teenager or in her 20’s, just like most of you.
So when you classify knitting as a “granny activity” you are not just insulting all of the literally millions of young people who knit, but you are also revealing the self-centered way in which you view your own grandmother: that everything she does is defined by and done for the benefit of her grandchildren.
The truth is, she probably learned to knit because she saw an awesome sweater or scarf and wanted to learn how to make it back when she was your age. It had nothing to do with you and nothing to do with being a grandmother.
Knitting is a creative activity, and creative people come in all ages, genders, sizes and colors. By saying that knitting is for grannies, you are saying that all the artistry and skill involved in knitting is not due to any talent or hard work on the part of the knitter; it’s just somehow a natural by-product of being old.
For the record, one of my grandmothers never knitted and never wanted to knit. When she got old, she used to volunteer doing other people’s taxes. So does that make doing your taxes a “granny thing”? I don’t think so.
EDIT: This also applies to crochet, embroidery, macrame, or anything else crafty.