I guess I wonder if this is a matter of the writer getting better at their craft, or is it the mainstreaming of their work and their need to now reach a broader audience? Or is it both? I don't have an answer. I'm just begging the question because honestly, the playwrights' writings you mentioned all get less and less interesting to me the later you get into their body of work. This is a matter of personal taste, obviously. I've often thought that playwrights' work gets less interesting the older they get, which seems to be the opposite of what you're saying. Again, it might just be me.
And as far as theaters craving young writers, what do you consider young? As you mentioned, 40 can be emerging. Disclosure: I work as a marketer of television so my opinion might be slightly skewed, but I think one of the reasons that theater isn't more popular is the institution's flat out refusal to keep up with popular culture. Personally, I like plays by young people, even if they're a little rough around the edges or ray (and sometimes because they are.) At least they're fresh.
I wanted to respond that with more than just another comment.
To the first point, it's both. As a person gets older, they become concerned with different things, different parts of life. That shows up in their work. Also, if they're a "working" playwright, they have to worry less about making a splash. They can write more confidently, with more assurance. I think that evens out their work. A contemporary of mine and I had a pretty depressing conversation about how we needed to write that play that would make middle-aged audience happy, which meant flattening out what we were doing. Both things are at work, I think. I agree that sometimes the plays get worse, but sometimes they get so much better. It also depends on the playwright. For me, in general, I do think they get deeper and stronger. Even the ones who don't head towards more "traditional" landscape. I love Baltimore Waltz, but I like the Long Christmas Ride Home or How I Learned to Drive more. I love Balm in Gilead, but I like Burn This more. It's a sign of the maturity of a playwright that they can handle structure and style with equal ease, in my opinion. But it is a matter of taste.
As for the notion of what's young in theatre...that's a messy, messy point that definitely leads to fighting. I do consider 40 to still be young in the world of playwriting, partly in comparison to our main audiences, partly because it's a long lead-time field. If I read an interesting news story and I want to write a play about it, chances are it's going to be, at best, two to three years before that play sees the light of day. In that time, there will be a Law & Order episode, a CSI episode, an SNL skit, a 30 Rock reference, a novel, a YouTube video and a feature film all about the same thing. I'll also be three years older. I wrote a play inspired by my life in my late twenties when I was in my late twenties. When it was finally given a workshop production, I was well into my thirties and the concerns of that play were way behind me. Certainly there are playwrights who move more quickly and get things up more quickly, but it's rare.
I don't know how much of theatre's popular culture gap has to do with a willful avoidance (though there's some of that snobbery to go around) or the long lead-time or the fact that we're often playing for an aging audience so we have to pitch our work at a shallower knowledge. But there is a gap. Theatres try to fill that gap by hiring "young" writers or writers who write "young" and that means plays that are less accomplished, less well-structured and, not to put a fine point on it, that might just be bad. I don't fault them for that, or fault the young writers. My concern is when we start holding up these plays as what's good. Or, what's even worse, in my mind, we just decide, without discussing it, that these plays are what's good and what people should be emulating. A little consciouness about what we're producing and why is a good thing.
I do also think that you can have both. Just having a bunch of references to Lady Gaga or Lindsay Lohan doesn't make something "young," or even pop culture-y. In general, theatre sometimes acts like a high school principal trying to connect with the students by wearing his cap backwards and dropping references that are three years out of date. We should be aiming for both, good plays that appeal to younger audiences, but with structure and character and stories that matter. I think it can be done.