“This Is Our Youth” is easily one of the most convincing plays I have ever seen. So good, in fact, that I actually saw it for the second time the other night. Granted the premise and concept may seem a bit painfully gen-X, but the dialogue and acting manage to so perfectly capture the essence of three very specific kinds of people that you can’t help but marvel at it. The play takes place in Dennis Ziegler’s Upper West Side bedroom, a sparsely decorated filthy studio paid for by his rich parents, who are most likely just happy to have him out of their own apartment. As Dennis struggles to figure out his life, bike messengering for drug money, the rest of the people from his privileged world are off at college and pursuing self-sufficient lives. Enter Dennis’ sloppy, shy and stoner friend Warren.