I am in my late thirties. (Don't tell anyone!) And I think my writing stabilized oh, a decade ago? I do not mean that it became perfect, just that I "found a voice", or a range of voices, that seem to work and that I have been using ever since. (My early Tolkien stuff, like Flawed and Fair and Gathering the Pieces, is from right after it happened. I don't think I could write those stories better now.) I believe I *have* learned a little more in the intervening, but it mostly has to do with how to self-edit better so I get the same result faster.

And of course it is possible that I will have some major shift in the future!