I think the Granny from Warner Bros. (the lady who owned Tweety) went a long way towards making people think knitting is for old people.
That same image came to my mind too.
Remember as we grew into our teens and early 20's a lot of us though that our parents were stupid? The next time you hear the comment " knitting is for old people", make the statement, "That goes to prove that the older people aren't as dumb as we grew up believing." Something on that line.