That’s what we were taught in herb class. Luckily, the most pervasive fern (bracken) is widely recognized as edible when fiddleheaded, and that’s what we ate.
Some species of ferns pull heavy metals out of the soil as they age, and are in fact used to clean up toxic waste, so eating an old fern, besides being unpalatable, is dangerous.
Some ferns contain enzymes that interfere with the human body’s uptake of essential vitamins/nutrients/etc; these enzymes may be present in greater potency in mature specimens, but they are not entirely absent in fiddleheads and can still cause a problem if they are consumed in quantity or over prolonged periods. Other ferns are toxic in different ways; bracken, although eaten, is apparently carcinogenic (cite).
Which is why I say that you should be quite certain about the identification of your target species when gathering wild foods of any kind. It makes sense anyway.
I’d never even heard of them until that Stephen King book, but they sound good. Don’t have 'em around West Texas, obviously. We have prickley pears, so, y’know, it’s a trade off.