Normally, the idea of being foolish isn't exactly a good thing. But then again, there's that saying, "ignorance is bliss." And maybe that's more applicable to the speaker of "Fern Hill," who's telling us about his full-of-promise youth. But no matter which way you slice it, you can't deny that this young prince was more than a little naïve. Sure, his youthful ignorance may have been a time of bliss, but it all comes crashing to an end, and we're thinking he probably should have seen that coming.
While the speaker enjoys being young and carefree, he realizes later that his youth was gone before he realized it was heading out in the first place. Hey, you snooze, you lose.
Even though the speaker later realizes youth passed him by, being carefree was its own reward, so he shouldn't be too bummed.