This is me trying to be helpful, it was covered in know your meme pretty well here: https://knowyourmeme.com/memes/avocado-toast
The term became a popular reference and in-joke for millennials after an article about millionaire Tim Gurner featured a quote from him saying he got rich by saving and not spending money on frivolous things such as avocado toast.
Edit: I don't think Gurner is a boomer judging by his pic. I can't find an age on him.
Edit: This page says he was born in 1982, so he's a gen x, I think. He's around 41 or 42. https://latestbollyholly.com/tim-gurner-wikipedia-age/