From what I found, leaves that are picked to make into food are best picked from spring to early summer. That way, they are still tender. They tend to get tougher and more fiberous as the summer goes on.
If they are going to be used for the flavor they infuse when you wrap a cheese in them, as opposed to eating them in a dish, I'm not sure that they need to be young, though younger leaves would be more supple and less likely to break or tear if wrapping a cheese. If they are boiled/blanched/marinated, etc before they are applied, I'm not sure this will make a difference either. I don't know much about the process, but I'd be interested in seeing the results.