Like Tim, I'm not sure about the notation, but "being young" is the subject of the sentence (as you can see in Tim's paraphrase), and is a gerund. Does that help? It might be easier to see the structure here: "What a misery studying can be!" "What a misery marriage can be!"

I guess the second one, I haven’t seen that notation. If you change the order and replace what with such, it would be “Being young can be such a misery.”