I can’t remember my exact date, but somewhere between my daughters’ being born, so late 1980s I.e. 37 or so years ago, which seems crazy in retrospect
I can clearly remember the final straw was grilling some lamb chops for my wife and I, and ensuring the tastiest way was to cook until they were still pink inside apologies if you’re grossed out - I am after all these years).
I suddenly realised what that actually meant and couldn’t eat them without gagging.
It had been building up for a while and I can thank a vegetarian work colleague (Susan - surname withheld), whom I originally disparaged, but began to respect for her lack of preaching*.
I stopped eating meat, but continued with fish for a few months (it made sense at the time!), before becoming firmly vegetarian.
*Good tip: don’t preach if you want to persuade someone else to become a vegetarian. Set an example, show them the great food you enjoy, etc.