I believe that Thrash is juvenilia, and the spirit that thrash projects is only possessed by those under 20 or so.
I think that people lose a certain spirit after 21
I think it varies with the people.
As far as thrash, nothing lasts forever -- it has to mature. It did with Four of a Kind and now it's time to update it, keeping the spirit alive.
The under-21s don't seem to be doing it, or with a few (like five max) notable exceptions, doing a goddamn thing of note (of quality).
Keep in mind that Bruckner, Beethoven, Schubert, and even Slayer wrote some of their best material after 21. Maturity isn't all bad. If you don't buy into bullshit definitions of maturity being wage-slavery and marrying an idiot and having idiot kids and keeping up with the Joneses, maturity can simply mean applying experience and knowledge to make a project that loses that juvenilia.
I think it varies with people as well, when I said after 21 I was just generalizing because thats the point where people tend to take their lives a bit more seriously, or at least take their non-seriousness more seriously.
Four of a Kind isn't usually considered to be on the same level as the dirty rotten LP or dealing with it for a reason, I don't really see it as a matured record, it just got updated with speed metal to make it more palatable. Which is unsurprising because like I said, the thrash spirit tends to fade in ones early 20s so. I know Slayer got even more thrashy after they hit 21 with RIB, but to me, it was an excellent record thats sadly a bit less convincing than No Mercy and Hell Awaits and as we know it was only downhill from there.
Of course those classic composers wrote some of their best material after 21, but I don't see why they're relevant because they weren't thrash and their music was aimed at a mature audience. As far as Slayer goes, I feel this supports my claim because their music started to become more and more artificial with every album after they hit 21. I don't buy into the modern definition of maturity and I don't see why you think I would, maturity is necessary part of life and just as maturity is important to have if one wants to create mature music, you must have an immature spirit to create immature music. Saying thrash should mature is like saying metal should be less contentious, it just defeats the purpose of thrash as a distinctive art form.