Actually on second thought the talis example is better than I first thought.
N.C. said the following earlier “No. We haven’t proven that minhag overrides halacha. We’ve proven that sometimes when we say minhag, we mean how you and your group hold in the halachah.”
Clearly nobody holds the halacha is a 13 year old CAN’T wear a talis! In other words nobody and no group says the halacha is a talis shouldnt be worn by 13 year olds.
Yet for most ashkenazic Jews we put off being mikayem a mitzvas aseh (granted its a mitzvah kiyumis and granted we dont put it off completely since we wear a talis katan) yet we remain as the M”B says “yoshev ubatil mimitzvas tzitzis” becasue that is our minhag! (again of course within the framework provided by the M”A we need (and have) Rishonim’s sanction for our minhag.
Yet I think this example proves that minhag does not mean ” how you and your group hold in the halachah”