All of my cabbage plants have bolted already, and did not form heads. Do you think because it got so hot a few weeks ago, that they just couldn't take the heat? Or did I just plant the wrong variety? I've read that you can/should still eat the leaves, but at some point they will become bitter. How do I know at what point in the bolting process they will be satisfying to eat?
Most of the cabbage family are biennials. This cycle has the plant grow leaves and roots the first year, overwinter, and bolt to flower and set seed the next spring. As humans adopted these plants, we have tried through breeding, to slow some of the floral triggers. They can't be eliminated since you would never have seed. But with cabbage, there is huge range of bolting sensitivity between varieties.
My sense is that one or all of the 2-3 hot day cycles of 80+ temperatures we had in mid-April, again in mid-May and again in early June triggered the bolting. For us, is was quite enjoyable but for a cabbage, it was a signal that it was time to bolt, something that otherwise would be delayed to 2022 in a normal spring. I wouldn't plant that particular cabbage again but instead look for ones with good bolt resistance. Catalogs should help you in that search. The cabbage is good to eat until it tastes bitter.